diff --git a/services/dummy-sink/.dockerignore b/services/dummy-sink/.dockerignore
new file mode 100644
index 0000000..05b8f29
--- /dev/null
+++ b/services/dummy-sink/.dockerignore
@@ -0,0 +1,4 @@
+**/*_test.go.bak
+**/.DS_Store
+**/*.swp
+**/*.swo
diff --git a/services/dummy-sink/Dockerfile b/services/dummy-sink/Dockerfile
new file mode 100644
index 0000000..a21daf3
--- /dev/null
+++ b/services/dummy-sink/Dockerfile
@@ -0,0 +1,36 @@
+# syntax=docker/dockerfile:1.7
+
+# ---------- build ----------
+FROM golang:1.23-alpine3.21 AS build
+
+ENV CGO_ENABLED=0 \
+    GOOS=linux \
+    GOFLAGS="-trimpath"
+
+WORKDIR /src
+
+COPY services/dummy-sink/go.mod ./services/dummy-sink/
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=cache,target=/go/pkg/mod \
+    cd services/dummy-sink && go mod download
+
+COPY services/dummy-sink/ ./services/dummy-sink/
+
+WORKDIR /src/services/dummy-sink
+RUN --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=cache,target=/go/pkg/mod \
+    go vet ./... && \
+    go test ./... && \
+    go build -ldflags="-s -w" -o /out/dummy-sink ./cmd/dummy-sink
+
+# ---------- runtime ----------
+# Distroless static+nonroot. The projected ServiceAccount token lives on a
+# tmpfs volume mounted by kubelet into /var/run/secrets/tokens/trust-api,
+# and /data/dummy-sink is a PVC mount — the container needs no shell.
+FROM gcr.io/distroless/static-debian12:nonroot
+
+COPY --from=build /out/dummy-sink /dummy-sink
+
+USER 65532:65532
+EXPOSE 8083
+ENTRYPOINT ["/dummy-sink"]
diff --git a/services/dummy-sink/README.md b/services/dummy-sink/README.md
new file mode 100644
index 0000000..6a4c787
--- /dev/null
+++ b/services/dummy-sink/README.md
@@ -0,0 +1,151 @@
+# dummy-sink
+
+Phase-1 Mock-Senke (`sink:dummy-sink`) per [DEV-611](/DEV/issues/DEV-611),
+parent [DEV-606](/DEV/issues/DEV-606).
+
+Trivial sink adapter that implements the §7 Senken-Vertrag of the
+architecture V1.1 at the minimum needed to close the Phase-1 trust chain:
+it accepts a decision, calls the verification-service, persists the
+payload to local storage, and emits a signed handover attestation.
+
+## Endpoint
+
+```
+POST /api/v1/sink/dummy/deliver
+Content-Type: application/json
+
+{
+  "decision_id":   "dec-123",
+  "payload_hash":  "sha256:<64-hex>",
+  "payload":       { ... },
+  "approval_ref":  "sha256:<64-hex>",
+  "transform_ref": "sha256:<64-hex>"
+}
+```
+
+`payload_hash` must match either `sha256(raw payload bytes)` or
+`sha256(canonical(payload))` — a verifier can reproduce either form.
+
+### Flow
+
+1. Verify the submitted hash matches `payload`.
+2. `POST {VERIFY_URL}/api/v1/verify/chain` with
+   `{decision_id, payload_hash, approval_ref, transform_ref, sink_target}`.
+3. Branch on the verdict:
+   - `valid: true` →
+     1. Write `payload` atomically to
+        `{DATA_DIR}/{decision_id}.json` (`.tmp` + rename).
+     2. Build the handover statement (predicateType
+        `https://basicstack.de/attestations/handover/v1`) that binds
+        `decision_id`, `payload_hash`, `approval_ref`, `transform_ref`,
+        and `sink_record_ref` into the trust chain.
+     3. `POST {SIGNING_URL}/api/v1/sign/dsse` with the double identity
+        headers (Pocket-ID bearer + projected SA token).
+     4. `POST {TLOG_URL}/api/v1/log/entries` with the signed envelope
+        + `content_hash`.
+     5. `201 Created { handover_ref, log_index, decision_id }`.
+   - `valid: false, replay: true` → `409 Conflict`.
+   - `valid: false` → `403 Forbidden { error: { reasons: [...] } }`.
+
+Replay protection intentionally lives in the verification-service
+(parent plan §Subtask 6); dummy-sink keeps no replay table. A second
+delivery with the same `decision_id` is funneled through the
+verification-service, which marks it as a replay and the sink translates
+that to `409`.
+
+## Non-scope
+
+- Real sinks (DB-App, SAP, webhooks) — all in Phase 4+.
+- No request authentication of the inbound caller; dummy-sink is reachable
+  only from in-cluster callers behind NetworkPolicy (Phase-1-minimum).
+- No local replay store; the verification-service owns `decision_id`
+  consumption.
+
+## Configuration
+
+| Flag / env                                      | Default                                       | Purpose                                                        |
+| ----------------------------------------------- | --------------------------------------------- | -------------------------------------------------------------- |
+| `-listen`                                       | `:8083`                                       | HTTP listen address.                                           |
+| `-data-dir`                                     | `/data/dummy-sink`                            | Where delivered payloads are written as `<decision_id>.json`.  |
+| `-verify-url`                                   | *(required)*                                  | Base URL of verification-service.                              |
+| `-signing-url`                                  | *(required)*                                  | Base URL of signing-service.                                   |
+| `-tlog-url`                                     | *(required)*                                  | Base URL of transparency-log.                                  |
+| `-sink-id`                                      | `sink:dummy-sink`                             | Logical sink identifier (used in verify + handover attestation). |
+| `-actor`                                        | `agent-dummy-sink`                            | Agent registry id of the dummy-sink identity.                  |
+| `-signing-key-ref`                              | `trust-api/agent-dummy-sink-2026-q4`          | `key_ref` the handover attestation is signed with.             |
+| `-pocket-id-issuer` (env `POCKET_ID_ISSUER`)    | *(required in prod)*                          | Pocket-ID OIDC issuer URL.                                     |
+| `-pocket-id-client-id` (env `POCKET_ID_CLIENT_ID`) | *(required in prod)*                       | OIDC client id for this agent.                                 |
+| `-pocket-id-client-secret` (env `POCKET_ID_CLIENT_SECRET`) | *(required in prod)*                | OIDC client secret for this agent.                             |
+| `-pocket-id-audience` (env `POCKET_ID_AUDIENCE`) | `trust-api`                                  | OIDC token audience.                                           |
+| `-workload-token-path` (env `DUMMY_SINK_WORKLOAD_TOKEN_PATH`) | `/var/run/secrets/tokens/trust-api` | Projected SA token path.                                       |
+| `-dev-pocket-id-token`                          | *(unset)*                                     | **Dev/CI only.** Static Pocket-ID access token.                |
+| `-dev-workload-token`                           | *(unset)*                                     | **Dev/CI only.** Static projected SA token.                    |
+
+## Handover attestation (`basicstack.de/attestations/handover/v1`)
+
+```jsonc
+{
+  "_type": "https://in-toto.io/Statement/v1",
+  "subject": [
+    {
+      "name": "handover/<decision_id>",
+      "digest": { "sha256": "<64-hex payload digest>" }
+    }
+  ],
+  "predicateType": "https://basicstack.de/attestations/handover/v1",
+  "predicate": {
+    "decision_id":     "<id>",
+    "payload_hash":    "sha256:<64-hex>",
+    "approval_ref":    "sha256:<64-hex>",
+    "transform_ref":   "sha256:<64-hex>",
+    "sink_id":         "sink:dummy-sink",
+    "sink_record_ref": "file://<DATA_DIR>/<decision_id>.json",
+    "materials": [
+      { "name": "approval/<approval_ref>",   "attestation_ref": "<approval_ref>" },
+      { "name": "transform/<transform_ref>", "attestation_ref": "<transform_ref>" }
+    ],
+    "actor":     "agent-dummy-sink",
+    "issued_at": "<RFC3339 UTC>"
+  }
+}
+```
+
+The subject digest is `sha256(payload)` so a verifier can detect any
+tampering between what the agent handed the sink and what the sink
+persisted. The two `materials[]` entries pin the attestation chain back
+to the (approval, transform) that authorised this handover.
+
+## Tests
+
+```
+cd services/dummy-sink
+go vet ./...
+go test ./...
+```
+
+The HTTP tests spin up the dummy-sink against httptest-backed fakes for
+verify / sign / tlog and cover the DEV-611 acceptance criteria:
+
+- **Happy path.** `valid=true` → `201` + file on disk + signer called with
+  the handover statement + TLog entry whose `content_hash` matches the
+  returned `handover_ref`.
+- **Chain invalid.** `valid=false` → `403` with `error.reasons`, no file
+  written, signer and TLog never called.
+- **Replay.** Second call with the same `decision_id` where the
+  verifier returns `replay: true` → `409`, signer not re-called.
+- **Payload hash mismatch.** Client-sent `payload_hash` disagrees with
+  both the raw and canonical hash → `400`, verifier never called.
+- **Bad request shapes.** Missing fields / malformed `sha256:` prefix →
+  `400`.
+- **Verifier unavailable / signer denied.** → `502` / `403` with the
+  appropriate code so operators can distinguish transport failures
+  from policy denials.
+
+## Known Phase-1 integration gap
+
+The testagent-transform CLI shipped in [DEV-610](/DEV/issues/DEV-610) still
+posts to `/api/v1/handover` with payload-in-body + `X-Attestation-Ref`
+header, whereas this sink implements the DEV-611 contract
+(`/api/v1/sink/dummy/deliver` with structured JSON). Aligning the agent
+is parent-plan Subtask 7 (`make phase1-smoke`); DEV-611 scope is sink-side
+only.
diff --git a/services/dummy-sink/cmd/dummy-sink/main.go b/services/dummy-sink/cmd/dummy-sink/main.go
new file mode 100644
index 0000000..a7ba9e6
--- /dev/null
+++ b/services/dummy-sink/cmd/dummy-sink/main.go
@@ -0,0 +1,171 @@
+// Command dummy-sink is the Phase-1 Mock-Senke (sink:dummy-sink) per
+// DEV-611. It receives a decision from the testagent-transform, calls
+// the verification-service to vet the (approval, transform) chain, and
+// — on valid=true — persists the payload to local storage and emits a
+// signed handover_attestation into the transparency-log.
+//
+// See services/dummy-sink/README.md for the full endpoint contract and
+// env configuration.
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/auth"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/httpapi"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/signer"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/sink"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/tlog"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/verify"
+)
+
+func main() {
+	listen := flag.String("listen", ":8083", "HTTP listen address")
+	dataDir := flag.String("data-dir", "/data/dummy-sink", "directory where delivered payloads are written as <decision_id>.json")
+
+	verifyURL := flag.String("verify-url", "", "base URL of verification-service (required)")
+	signingURL := flag.String("signing-url", "", "base URL of signing-service (required)")
+	tlogURL := flag.String("tlog-url", "", "base URL of transparency-log (required)")
+
+	sinkID := flag.String("sink-id", "sink:dummy-sink", "logical sink identifier used in verify + handover attestation")
+	actor := flag.String("actor", "agent-dummy-sink", "agent registry id of the dummy-sink identity")
+	keyRef := flag.String("signing-key-ref", "trust-api/agent-dummy-sink-2026-q4", "key_ref the handover attestation is signed with")
+
+	// Pocket-ID OIDC client_credentials for the Authorization: Bearer side
+	// of the signing-service double-identity check.
+	pocketIDIssuer := flag.String("pocket-id-issuer", os.Getenv("POCKET_ID_ISSUER"), "Pocket-ID OIDC issuer URL (env POCKET_ID_ISSUER)")
+	pocketIDClientID := flag.String("pocket-id-client-id", os.Getenv("POCKET_ID_CLIENT_ID"), "Pocket-ID client id (env POCKET_ID_CLIENT_ID)")
+	pocketIDClientSecret := flag.String("pocket-id-client-secret", os.Getenv("POCKET_ID_CLIENT_SECRET"), "Pocket-ID client secret (env POCKET_ID_CLIENT_SECRET)")
+	pocketIDAudience := flag.String("pocket-id-audience", envOr("POCKET_ID_AUDIENCE", "trust-api"), "Pocket-ID token audience (env POCKET_ID_AUDIENCE)")
+
+	// Projected ServiceAccount token for the X-Workload-Token side.
+	workloadTokenPath := flag.String("workload-token-path", envOr("DUMMY_SINK_WORKLOAD_TOKEN_PATH", "/var/run/secrets/tokens/trust-api"),
+		"path to the projected SA token file (env DUMMY_SINK_WORKLOAD_TOKEN_PATH)")
+
+	// Optional static tokens for dev/CI where Pocket-ID / kubelet are not
+	// wired up. Mutually exclusive with the real sources.
+	staticPocketIDToken := flag.String("dev-pocket-id-token", "", "static Pocket-ID access token for dev/CI only")
+	staticWorkloadToken := flag.String("dev-workload-token", "", "static projected SA token for dev/CI only")
+
+	flag.Parse()
+
+	logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
+
+	if err := requireNonEmpty("verify-url", *verifyURL); err != nil {
+		exit(logger, err)
+	}
+	if err := requireNonEmpty("signing-url", *signingURL); err != nil {
+		exit(logger, err)
+	}
+	if err := requireNonEmpty("tlog-url", *tlogURL); err != nil {
+		exit(logger, err)
+	}
+
+	pocketIDSource, err := buildPocketIDSource(*staticPocketIDToken, *pocketIDIssuer, *pocketIDClientID, *pocketIDClientSecret, *pocketIDAudience)
+	if err != nil {
+		exit(logger, err)
+	}
+	workloadSource, err := buildWorkloadSource(*staticWorkloadToken, *workloadTokenPath)
+	if err != nil {
+		exit(logger, err)
+	}
+
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	srv := httpapi.Server{
+		Verifier: verify.Client{BaseURL: *verifyURL},
+		Store:    sink.Store{DataDir: *dataDir},
+		Signer: signer.Client{
+			BaseURL:       *signingURL,
+			PocketIDToken: pocketIDSource,
+			WorkloadToken: workloadSource,
+		},
+		TLog:          tlog.Client{BaseURL: *tlogURL},
+		SinkID:        *sinkID,
+		Actor:         *actor,
+		SigningKeyRef: *keyRef,
+		Logger:        logger,
+		Now:           time.Now,
+	}
+
+	mux := http.NewServeMux()
+	srv.Mount(mux)
+
+	hsrv := &http.Server{
+		Addr:              *listen,
+		Handler:           mux,
+		ReadHeaderTimeout: 5 * time.Second,
+	}
+	go func() {
+		logger.Info("dummy-sink listening",
+			"addr", *listen,
+			"data_dir", *dataDir,
+			"sink_id", *sinkID,
+			"actor", *actor,
+			"signing_key_ref", *keyRef,
+			"verify_url", *verifyURL,
+			"signing_url", *signingURL,
+			"tlog_url", *tlogURL,
+		)
+		if err := hsrv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			logger.Error("http serve", "err", err)
+		}
+	}()
+	<-ctx.Done()
+	shutdownCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	_ = hsrv.Shutdown(shutdownCtx)
+}
+
+func buildPocketIDSource(staticTok, issuer, clientID, clientSecret, audience string) (auth.TokenSource, error) {
+	if staticTok != "" {
+		return auth.StaticToken(staticTok), nil
+	}
+	if issuer == "" || clientID == "" || clientSecret == "" {
+		return nil, fmt.Errorf("either -dev-pocket-id-token or (-pocket-id-issuer + -pocket-id-client-id + -pocket-id-client-secret) is required")
+	}
+	return &auth.PocketIDTokenSource{
+		Issuer:       issuer,
+		ClientID:     clientID,
+		ClientSecret: clientSecret,
+		Audience:     audience,
+	}, nil
+}
+
+func buildWorkloadSource(staticTok, path string) (auth.TokenSource, error) {
+	if staticTok != "" {
+		return auth.StaticToken(staticTok), nil
+	}
+	if path == "" {
+		return nil, fmt.Errorf("either -dev-workload-token or -workload-token-path is required")
+	}
+	return &auth.ProjectedSATokenSource{Path: path}, nil
+}
+
+func requireNonEmpty(name, value string) error {
+	if value == "" {
+		return fmt.Errorf("-%s is required", name)
+	}
+	return nil
+}
+
+func envOr(name, fallback string) string {
+	if v := os.Getenv(name); v != "" {
+		return v
+	}
+	return fallback
+}
+
+func exit(logger *slog.Logger, err error) {
+	logger.Error("dummy-sink init", "err", err)
+	os.Exit(1)
+}
diff --git a/services/dummy-sink/go.mod b/services/dummy-sink/go.mod
new file mode 100644
index 0000000..539e856
--- /dev/null
+++ b/services/dummy-sink/go.mod
@@ -0,0 +1,3 @@
+module basicstack.de/proofedmesh/services/dummy-sink
+
+go 1.23
diff --git a/services/dummy-sink/internal/auth/pocket_id.go b/services/dummy-sink/internal/auth/pocket_id.go
new file mode 100644
index 0000000..db59e37
--- /dev/null
+++ b/services/dummy-sink/internal/auth/pocket_id.go
@@ -0,0 +1,165 @@
+package auth
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+	"sync"
+	"time"
+)
+
+// PocketIDTokenSource issues Bearer tokens using the Pocket-ID OIDC
+// `client_credentials` grant and caches each token until shortly before
+// its expiry. Safe for concurrent use.
+//
+// Shape matches services/testagent-transform/internal/auth so the
+// signing-service's Pocket-ID validator sees the same token format
+// admin-api, approval-service, and testagent-transform already send.
+type PocketIDTokenSource struct {
+	Issuer        string
+	ClientID      string
+	ClientSecret  string
+	Scope         string
+	Audience      string
+	Client        *http.Client
+	RefreshLeeway time.Duration
+	Now           func() time.Time
+
+	mu            sync.Mutex
+	tokenEndpoint string
+	cachedToken   string
+	cachedExpiry  time.Time
+}
+
+type pocketIDTokenResponse struct {
+	AccessToken string `json:"access_token"`
+	TokenType   string `json:"token_type"`
+	ExpiresIn   int    `json:"expires_in"`
+}
+
+type pocketIDDiscoveryDoc struct {
+	TokenEndpoint string `json:"token_endpoint"`
+}
+
+// Token returns a cached access token when still valid, or fetches a fresh
+// one from Pocket-ID.
+func (p *PocketIDTokenSource) Token(ctx context.Context) (string, error) {
+	if p == nil {
+		return "", errors.New("auth.PocketIDTokenSource: nil receiver")
+	}
+	if p.Issuer == "" {
+		return "", errors.New("auth.PocketIDTokenSource: Issuer is required")
+	}
+	if p.ClientID == "" || p.ClientSecret == "" {
+		return "", errors.New("auth.PocketIDTokenSource: ClientID and ClientSecret are required")
+	}
+
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	leeway := p.RefreshLeeway
+	if leeway <= 0 {
+		leeway = 30 * time.Second
+	}
+	now := p.nowLocked()
+	if p.cachedToken != "" && now.Add(leeway).Before(p.cachedExpiry) {
+		return p.cachedToken, nil
+	}
+
+	if p.tokenEndpoint == "" {
+		if err := p.discoverLocked(ctx); err != nil {
+			return "", err
+		}
+	}
+	tok, expiry, err := p.fetchLocked(ctx, now)
+	if err != nil {
+		return "", err
+	}
+	p.cachedToken = tok
+	p.cachedExpiry = expiry
+	return tok, nil
+}
+
+func (p *PocketIDTokenSource) nowLocked() time.Time {
+	if p.Now != nil {
+		return p.Now()
+	}
+	return time.Now()
+}
+
+func (p *PocketIDTokenSource) httpClient() *http.Client {
+	if p.Client != nil {
+		return p.Client
+	}
+	return &http.Client{Timeout: 10 * time.Second}
+}
+
+func (p *PocketIDTokenSource) discoverLocked(ctx context.Context) error {
+	base := strings.TrimRight(p.Issuer, "/")
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, base+"/.well-known/openid-configuration", nil)
+	if err != nil {
+		return fmt.Errorf("pocket id discovery: %w", err)
+	}
+	req.Header.Set("Accept", "application/json")
+	resp, err := p.httpClient().Do(req)
+	if err != nil {
+		return fmt.Errorf("pocket id discovery: %w", err)
+	}
+	defer resp.Body.Close()
+	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("pocket id discovery: status %d: %s", resp.StatusCode, string(raw))
+	}
+	var doc pocketIDDiscoveryDoc
+	if err := json.Unmarshal(raw, &doc); err != nil {
+		return fmt.Errorf("pocket id discovery: decode: %w", err)
+	}
+	if doc.TokenEndpoint == "" {
+		return errors.New("pocket id discovery: token_endpoint missing from OIDC configuration")
+	}
+	p.tokenEndpoint = doc.TokenEndpoint
+	return nil
+}
+
+func (p *PocketIDTokenSource) fetchLocked(ctx context.Context, now time.Time) (string, time.Time, error) {
+	form := url.Values{}
+	form.Set("grant_type", "client_credentials")
+	if p.Scope != "" {
+		form.Set("scope", p.Scope)
+	}
+	if p.Audience != "" {
+		form.Set("audience", p.Audience)
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, p.tokenEndpoint, strings.NewReader(form.Encode()))
+	if err != nil {
+		return "", time.Time{}, fmt.Errorf("pocket id token: %w", err)
+	}
+	req.SetBasicAuth(p.ClientID, p.ClientSecret)
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Set("Accept", "application/json")
+	resp, err := p.httpClient().Do(req)
+	if err != nil {
+		return "", time.Time{}, fmt.Errorf("pocket id token: %w", err)
+	}
+	defer resp.Body.Close()
+	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	if resp.StatusCode != http.StatusOK {
+		return "", time.Time{}, fmt.Errorf("pocket id token: status %d: %s", resp.StatusCode, string(raw))
+	}
+	var out pocketIDTokenResponse
+	if err := json.Unmarshal(raw, &out); err != nil {
+		return "", time.Time{}, fmt.Errorf("pocket id token: decode: %w", err)
+	}
+	if out.AccessToken == "" {
+		return "", time.Time{}, errors.New("pocket id token: response missing access_token")
+	}
+	if out.ExpiresIn <= 0 {
+		return "", time.Time{}, errors.New("pocket id token: response has non-positive expires_in")
+	}
+	return out.AccessToken, now.Add(time.Duration(out.ExpiresIn) * time.Second), nil
+}
diff --git a/services/dummy-sink/internal/auth/sa_token.go b/services/dummy-sink/internal/auth/sa_token.go
new file mode 100644
index 0000000..4ef00c4
--- /dev/null
+++ b/services/dummy-sink/internal/auth/sa_token.go
@@ -0,0 +1,67 @@
+package auth
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"strings"
+	"sync"
+	"time"
+)
+
+// ProjectedSATokenSource reads the kubelet-written projected
+// ServiceAccount token from a mounted Volume (typically
+// `/var/run/secrets/tokens/trust-api`) and re-reads it on each call so
+// kubelet's periodic refresh is picked up without a service restart.
+//
+// A short in-process cache avoids a syscall on every signing-service call;
+// kubelet refreshes projected tokens well before their `exp`, so a few
+// seconds of staleness is fine. Set CacheTTL negative to disable.
+type ProjectedSATokenSource struct {
+	Path     string
+	CacheTTL time.Duration
+	Now      func() time.Time
+
+	mu          sync.Mutex
+	cachedToken string
+	cachedAt    time.Time
+}
+
+// Token returns the current projected ServiceAccount token.
+func (p *ProjectedSATokenSource) Token(_ context.Context) (string, error) {
+	if p == nil {
+		return "", fmt.Errorf("auth.ProjectedSATokenSource: nil receiver")
+	}
+	if p.Path == "" {
+		return "", fmt.Errorf("auth.ProjectedSATokenSource: Path is required")
+	}
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	ttl := p.CacheTTL
+	if ttl == 0 {
+		ttl = 30 * time.Second
+	}
+	now := p.now()
+	if ttl > 0 && p.cachedToken != "" && now.Sub(p.cachedAt) < ttl {
+		return p.cachedToken, nil
+	}
+	raw, err := os.ReadFile(p.Path)
+	if err != nil {
+		return "", fmt.Errorf("read projected SA token %q: %w", p.Path, err)
+	}
+	tok := strings.TrimSpace(string(raw))
+	if tok == "" {
+		return "", fmt.Errorf("projected SA token at %q is empty", p.Path)
+	}
+	p.cachedToken = tok
+	p.cachedAt = now
+	return tok, nil
+}
+
+func (p *ProjectedSATokenSource) now() time.Time {
+	if p.Now != nil {
+		return p.Now()
+	}
+	return time.Now()
+}
diff --git a/services/dummy-sink/internal/auth/token_source.go b/services/dummy-sink/internal/auth/token_source.go
new file mode 100644
index 0000000..df55a71
--- /dev/null
+++ b/services/dummy-sink/internal/auth/token_source.go
@@ -0,0 +1,33 @@
+// Package auth exposes the two identity sources dummy-sink uses to call
+// the signing-service:
+//
+//  1. Pocket-ID client_credentials access token (Authorization: Bearer …)
+//  2. Projected ServiceAccount token (X-Workload-Token …) read from a
+//     kubelet-managed Volume (`projected: { serviceAccountToken: … }`).
+//
+// Both are modelled behind the same TokenSource interface so the
+// orchestration code never cares about the identity's origin. The split
+// mirrors services/testagent-transform/internal/auth and services/
+// approval-service/internal/signer; both of those services already use
+// the double-identity flow against signing-service.
+package auth
+
+import "context"
+
+// TokenSource returns a bearer-style token. Implementations may cache
+// internally; the caller is expected to retry on error.
+type TokenSource interface {
+	Token(ctx context.Context) (string, error)
+}
+
+// StaticToken is a constant TokenSource. Primarily used in tests.
+type StaticToken string
+
+// Token returns s.
+func (s StaticToken) Token(_ context.Context) (string, error) { return string(s), nil }
+
+// TokenFunc adapts a plain function to TokenSource.
+type TokenFunc func(ctx context.Context) (string, error)
+
+// Token calls f.
+func (f TokenFunc) Token(ctx context.Context) (string, error) { return f(ctx) }
diff --git a/services/dummy-sink/internal/canonical/canonical.go b/services/dummy-sink/internal/canonical/canonical.go
new file mode 100644
index 0000000..0fe6f7c
--- /dev/null
+++ b/services/dummy-sink/internal/canonical/canonical.go
@@ -0,0 +1,103 @@
+// Package canonical implements the deterministic JSON serialisation
+// (sorted object keys) dummy-sink uses when it computes payload / subject
+// digests that MUST reproduce byte-for-byte on the verifier side.
+//
+// MUST stay compatible with:
+//   - services/approval-service/internal/canonical/canonical.go
+//   - services/signing-service/internal/canonical/canonical.go
+//   - services/testagent-transform/internal/canonical/canonical.go
+package canonical
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"sort"
+)
+
+// SHA256Hex returns the hex-encoded SHA-256 of b.
+func SHA256Hex(b []byte) string {
+	sum := sha256.Sum256(b)
+	return hex.EncodeToString(sum[:])
+}
+
+// Marshal returns the canonical JSON encoding of v (sorted object keys).
+func Marshal(v any) ([]byte, error) {
+	first, err := json.Marshal(v)
+	if err != nil {
+		return nil, err
+	}
+	var any_ any
+	if err := json.Unmarshal(first, &any_); err != nil {
+		return nil, err
+	}
+	var buf bytes.Buffer
+	if err := writeSorted(&buf, any_); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+// Hash returns the sha256:<hex> content hash of v's canonical JSON along
+// with the raw canonical bytes.
+func Hash(v any) (string, []byte, error) {
+	raw, err := Marshal(v)
+	if err != nil {
+		return "", nil, err
+	}
+	return "sha256:" + SHA256Hex(raw), raw, nil
+}
+
+func writeSorted(buf *bytes.Buffer, v any) error {
+	switch x := v.(type) {
+	case nil:
+		buf.WriteString("null")
+	case bool:
+		if x {
+			buf.WriteString("true")
+		} else {
+			buf.WriteString("false")
+		}
+	case float64:
+		b, _ := json.Marshal(x)
+		buf.Write(b)
+	case string:
+		b, _ := json.Marshal(x)
+		buf.Write(b)
+	case []any:
+		buf.WriteByte('[')
+		for i, el := range x {
+			if i > 0 {
+				buf.WriteByte(',')
+			}
+			if err := writeSorted(buf, el); err != nil {
+				return err
+			}
+		}
+		buf.WriteByte(']')
+	case map[string]any:
+		keys := make([]string, 0, len(x))
+		for k := range x {
+			keys = append(keys, k)
+		}
+		sort.Strings(keys)
+		buf.WriteByte('{')
+		for i, k := range keys {
+			if i > 0 {
+				buf.WriteByte(',')
+			}
+			kb, _ := json.Marshal(k)
+			buf.Write(kb)
+			buf.WriteByte(':')
+			if err := writeSorted(buf, x[k]); err != nil {
+				return err
+			}
+		}
+		buf.WriteByte('}')
+	default:
+		return fmt.Errorf("canonical.writeSorted: unsupported type %T", v)
+	}
+	return nil
+}
diff --git a/services/dummy-sink/internal/dsse/dsse.go b/services/dummy-sink/internal/dsse/dsse.go
new file mode 100644
index 0000000..ad29121
--- /dev/null
+++ b/services/dummy-sink/internal/dsse/dsse.go
@@ -0,0 +1,22 @@
+// Package dsse models the DSSE envelope shape the signing-service
+// produces. dummy-sink does not verify envelopes locally; it simply ships
+// the envelope the signing-service returned to the transparency-log.
+//
+// Shape mirrors services/testagent-transform/internal/dsse.Envelope so a
+// future verifier can round-trip the handover envelope through either
+// package byte-for-byte.
+package dsse
+
+// EnvelopeSignature mirrors the DSSE spec.
+type EnvelopeSignature struct {
+	KeyID string `json:"keyid"`
+	Sig   string `json:"sig"`
+}
+
+// Envelope is the DSSE envelope the signing-service returns. Payload is
+// base64url (no padding) of the canonical in-toto Statement bytes.
+type Envelope struct {
+	PayloadType string              `json:"payloadType"`
+	Payload     string              `json:"payload"`
+	Signatures  []EnvelopeSignature `json:"signatures"`
+}
diff --git a/services/dummy-sink/internal/handover/builder.go b/services/dummy-sink/internal/handover/builder.go
new file mode 100644
index 0000000..19db5a1
--- /dev/null
+++ b/services/dummy-sink/internal/handover/builder.go
@@ -0,0 +1,95 @@
+// Package handover builds the in-toto v1 Statement for the Phase-1
+// handover attestation (predicateType =
+// https://basicstack.de/attestations/handover/v1). The attestation
+// records that `sink:dummy-sink` persisted `payload` (identified by
+// `payload_hash`) on behalf of the (approval_ref, transform_ref) chain,
+// per §7.0 of the architecture.
+package handover
+
+import (
+	"errors"
+	"time"
+
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/signer"
+)
+
+// PredicateType is the stable URL the signing-service, verification
+// chain, and transparency-log key the handover statement off of.
+const PredicateType = "https://basicstack.de/attestations/handover/v1"
+
+// Input captures everything the sink knows about a successfully
+// persisted delivery.
+type Input struct {
+	DecisionID   string
+	PayloadHash  string // sha256:<hex>
+	ApprovalRef  string
+	TransformRef string
+
+	SinkID        string // e.g. "sink:dummy-sink"
+	SinkRecordRef string // storage locator (e.g. "file:///data/dummy-sink/<decision_id>.json")
+	Actor         string // agent registry id of the dummy-sink identity
+
+	IssuedAt time.Time
+}
+
+// Build returns the in-toto Statement handed to the signing-service.
+// The subject binds the attestation to the payload hash so a downstream
+// verifier can detect any tampering between what the agent sent and what
+// the sink persisted.
+func Build(in Input) (signer.Statement, error) {
+	if in.DecisionID == "" || in.PayloadHash == "" ||
+		in.ApprovalRef == "" || in.TransformRef == "" ||
+		in.SinkID == "" || in.SinkRecordRef == "" || in.Actor == "" {
+		return nil, errors.New("handover.Build: Input missing required field")
+	}
+	hashHex, err := StripSha256Prefix(in.PayloadHash)
+	if err != nil {
+		return nil, err
+	}
+	issuedAt := in.IssuedAt
+	if issuedAt.IsZero() {
+		issuedAt = time.Now()
+	}
+	return signer.Statement{
+		"_type": "https://in-toto.io/Statement/v1",
+		"subject": []any{
+			map[string]any{
+				"name": "handover/" + in.DecisionID,
+				"digest": map[string]any{
+					"sha256": hashHex,
+				},
+			},
+		},
+		"predicateType": PredicateType,
+		"predicate": map[string]any{
+			"decision_id":     in.DecisionID,
+			"payload_hash":    in.PayloadHash,
+			"approval_ref":    in.ApprovalRef,
+			"transform_ref":   in.TransformRef,
+			"sink_id":         in.SinkID,
+			"sink_record_ref": in.SinkRecordRef,
+			"materials": []any{
+				map[string]any{
+					"name":            "approval/" + in.ApprovalRef,
+					"attestation_ref": in.ApprovalRef,
+				},
+				map[string]any{
+					"name":            "transform/" + in.TransformRef,
+					"attestation_ref": in.TransformRef,
+				},
+			},
+			"actor":     in.Actor,
+			"issued_at": issuedAt.UTC().Format(time.RFC3339),
+		},
+	}, nil
+}
+
+// StripSha256Prefix normalises a `sha256:<hex>` hash to the bare hex
+// subject-digest form in-toto uses.
+func StripSha256Prefix(hash string) (string, error) {
+	const prefix = "sha256:"
+	if len(hash) <= len(prefix) || hash[:len(prefix)] != prefix {
+		return "", errors.New("handover: payload_hash must be sha256:<hex>")
+	}
+	return hash[len(prefix):], nil
+}
diff --git a/services/dummy-sink/internal/handover/builder_test.go b/services/dummy-sink/internal/handover/builder_test.go
new file mode 100644
index 0000000..b0fca52
--- /dev/null
+++ b/services/dummy-sink/internal/handover/builder_test.go
@@ -0,0 +1,69 @@
+package handover
+
+import (
+	"testing"
+	"time"
+)
+
+func TestBuildHappyPath(t *testing.T) {
+	in := Input{
+		DecisionID:    "dec-1",
+		PayloadHash:   "sha256:aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
+		ApprovalRef:   "sha256:bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb",
+		TransformRef:  "sha256:cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc",
+		SinkID:        "sink:dummy-sink",
+		SinkRecordRef: "file:///data/dummy-sink/dec-1.json",
+		Actor:         "agent-dummy-sink",
+		IssuedAt:      time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC),
+	}
+	stmt, err := Build(in)
+	if err != nil {
+		t.Fatalf("Build: %v", err)
+	}
+	if stmt["predicateType"] != PredicateType {
+		t.Fatalf("predicateType = %v, want %v", stmt["predicateType"], PredicateType)
+	}
+	pred := stmt["predicate"].(map[string]any)
+	if pred["decision_id"] != "dec-1" {
+		t.Fatalf("predicate.decision_id = %v", pred["decision_id"])
+	}
+	if pred["approval_ref"] != in.ApprovalRef {
+		t.Fatalf("predicate.approval_ref = %v", pred["approval_ref"])
+	}
+	if pred["issued_at"] != "2026-01-02T03:04:05Z" {
+		t.Fatalf("predicate.issued_at = %v", pred["issued_at"])
+	}
+	subject := stmt["subject"].([]any)[0].(map[string]any)
+	digest := subject["digest"].(map[string]any)
+	wantHex := "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+	if digest["sha256"] != wantHex {
+		t.Fatalf("digest.sha256 = %v, want %v", digest["sha256"], wantHex)
+	}
+	materials := pred["materials"].([]any)
+	if len(materials) != 2 {
+		t.Fatalf("materials len = %d, want 2", len(materials))
+	}
+}
+
+func TestBuildRequiresFields(t *testing.T) {
+	_, err := Build(Input{})
+	if err == nil {
+		t.Fatalf("Build empty: want err")
+	}
+}
+
+func TestStripSha256Prefix(t *testing.T) {
+	hex, err := StripSha256Prefix("sha256:abcd")
+	if err != nil {
+		t.Fatalf("StripSha256Prefix: %v", err)
+	}
+	if hex != "abcd" {
+		t.Fatalf("hex = %q, want abcd", hex)
+	}
+	if _, err := StripSha256Prefix("sha512:abcd"); err == nil {
+		t.Fatalf("sha512 prefix should error")
+	}
+	if _, err := StripSha256Prefix(""); err == nil {
+		t.Fatalf("empty should error")
+	}
+}
diff --git a/services/dummy-sink/internal/httpapi/server.go b/services/dummy-sink/internal/httpapi/server.go
new file mode 100644
index 0000000..7aeed20
--- /dev/null
+++ b/services/dummy-sink/internal/httpapi/server.go
@@ -0,0 +1,286 @@
+// Package httpapi exposes dummy-sink's single endpoint:
+// POST /api/v1/sink/dummy/deliver per the DEV-611 contract.
+//
+// Flow:
+//  1. Parse the request body ({decision_id, payload_hash, payload,
+//     approval_ref, transform_ref}).
+//  2. Call verification-service /api/v1/verify/chain.
+//  3. On valid=true:
+//     a. Write payload atomically to <DataDir>/<decision_id>.json.
+//     b. Build the handover_attestation (schema
+//        basicstack.de/attestations/handover/v1).
+//     c. Sign it via signing-service.
+//     d. Submit the DSSE envelope to the transparency-log.
+//     e. Reply 201 Created {handover_ref, log_index}.
+//  4. On valid=false:
+//     a. If verification-service flagged `replay: true`, reply 409.
+//     b. Otherwise reply 403 Forbidden {reasons: [...]}.
+//
+// Replay protection intentionally lives in the verification-service per
+// the parent plan: dummy-sink keeps no replay table of its own.
+package httpapi
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"log/slog"
+	"net/http"
+	"strings"
+	"time"
+
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/canonical"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/dsse"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/handover"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/signer"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/sink"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/tlog"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/verify"
+)
+
+// Verifier calls the verification-service.
+type Verifier interface {
+	Verify(ctx context.Context, req verify.Request) (verify.Result, error)
+}
+
+// Signer calls the signing-service.
+type Signer interface {
+	Sign(ctx context.Context, keyRef string, stmt signer.Statement) (signer.SignedResult, error)
+}
+
+// TLog submits DSSE envelopes to the transparency-log.
+type TLog interface {
+	Submit(ctx context.Context, env dsse.Envelope, contentHash string) (tlog.Receipt, error)
+}
+
+// Store writes payloads to disk atomically.
+type Store interface {
+	Write(decisionID string, payload any) (sink.WriteResult, error)
+}
+
+// Server wires the delivery pipeline.
+type Server struct {
+	Verifier      Verifier
+	Store         Store
+	Signer        Signer
+	TLog          TLog
+	SinkID        string // e.g. "sink:dummy-sink"
+	Actor         string // agent registry id of the dummy-sink identity
+	SigningKeyRef string // e.g. "trust-api/agent-dummy-sink-2026-q4"
+
+	Logger *slog.Logger
+	Now    func() time.Time
+}
+
+// Mount registers the endpoints on mux.
+func (s Server) Mount(mux *http.ServeMux) {
+	mux.HandleFunc("POST /api/v1/sink/dummy/deliver", s.handleDeliver)
+	mux.HandleFunc("GET /healthz", s.handleHealth)
+}
+
+type deliverRequest struct {
+	DecisionID   string          `json:"decision_id"`
+	PayloadHash  string          `json:"payload_hash"`
+	Payload      json.RawMessage `json:"payload"`
+	ApprovalRef  string          `json:"approval_ref"`
+	TransformRef string          `json:"transform_ref"`
+}
+
+type deliverResponse struct {
+	HandoverRef string `json:"handover_ref"`
+	LogIndex    int64  `json:"log_index"`
+	DecisionID  string `json:"decision_id"`
+}
+
+type errResponse struct {
+	Error errBody `json:"error"`
+}
+
+type errBody struct {
+	Code    string   `json:"code"`
+	Message string   `json:"message"`
+	Reasons []string `json:"reasons,omitempty"`
+}
+
+func (s Server) handleHealth(w http.ResponseWriter, _ *http.Request) {
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte(`{"status":"ok"}`))
+}
+
+func (s Server) handleDeliver(w http.ResponseWriter, r *http.Request) {
+	logger := s.logger()
+	ctx := r.Context()
+
+	raw, err := io.ReadAll(io.LimitReader(r.Body, 10<<20))
+	if err != nil {
+		writeErr(w, http.StatusBadRequest, "invalid_body", "read body: "+err.Error(), nil)
+		return
+	}
+	defer r.Body.Close()
+
+	var req deliverRequest
+	if err := json.Unmarshal(raw, &req); err != nil {
+		writeErr(w, http.StatusBadRequest, "invalid_body", "parse body: "+err.Error(), nil)
+		return
+	}
+	if err := validateRequest(req); err != nil {
+		writeErr(w, http.StatusBadRequest, "invalid_body", err.Error(), nil)
+		return
+	}
+
+	payloadRaw := []byte(req.Payload)
+	rawDigest := sha256.Sum256(payloadRaw)
+	rawHex := "sha256:" + hex.EncodeToString(rawDigest[:])
+
+	var payloadAny any
+	if err := json.Unmarshal(payloadRaw, &payloadAny); err != nil {
+		writeErr(w, http.StatusBadRequest, "invalid_body", "payload must be valid JSON: "+err.Error(), nil)
+		return
+	}
+	canonicalHash, _, err := canonical.Hash(payloadAny)
+	if err != nil {
+		writeErr(w, http.StatusBadRequest, "invalid_body", "canonicalise payload: "+err.Error(), nil)
+		return
+	}
+	if req.PayloadHash != rawHex && req.PayloadHash != canonicalHash {
+		writeErr(w, http.StatusBadRequest, "payload_hash_mismatch",
+			fmt.Sprintf("payload_hash %q matches neither raw (%s) nor canonical (%s)",
+				req.PayloadHash, rawHex, canonicalHash), nil)
+		return
+	}
+
+	verdict, err := s.Verifier.Verify(ctx, verify.Request{
+		DecisionID:   req.DecisionID,
+		PayloadHash:  req.PayloadHash,
+		ApprovalRef:  req.ApprovalRef,
+		TransformRef: req.TransformRef,
+		SinkTarget:   s.SinkID,
+	})
+	if err != nil {
+		logger.Error("verify.chain failed", "decision_id", req.DecisionID, "err", err)
+		if errors.Is(err, verify.ErrVerifyUnavailable) {
+			writeErr(w, http.StatusBadGateway, "verify_unavailable", err.Error(), nil)
+			return
+		}
+		writeErr(w, http.StatusBadGateway, "verify_failed", err.Error(), nil)
+		return
+	}
+
+	if !verdict.Valid {
+		if verdict.Replay {
+			writeErr(w, http.StatusConflict, "replay", "decision_id already consumed", verdict.Reasons)
+			return
+		}
+		writeErr(w, http.StatusForbidden, "chain_invalid", "verification chain rejected delivery", verdict.Reasons)
+		return
+	}
+
+	write, err := s.Store.Write(req.DecisionID, payloadAny)
+	if err != nil {
+		logger.Error("store.Write failed", "decision_id", req.DecisionID, "err", err)
+		if errors.Is(err, sink.ErrInvalidDecisionID) {
+			writeErr(w, http.StatusBadRequest, "invalid_decision_id", err.Error(), nil)
+			return
+		}
+		writeErr(w, http.StatusInternalServerError, "sink_write_failed", err.Error(), nil)
+		return
+	}
+
+	stmt, err := handover.Build(handover.Input{
+		DecisionID:    req.DecisionID,
+		PayloadHash:   req.PayloadHash,
+		ApprovalRef:   req.ApprovalRef,
+		TransformRef:  req.TransformRef,
+		SinkID:        s.SinkID,
+		SinkRecordRef: "file://" + write.Path,
+		Actor:         s.Actor,
+		IssuedAt:      s.now(),
+	})
+	if err != nil {
+		logger.Error("handover.Build failed", "decision_id", req.DecisionID, "err", err)
+		writeErr(w, http.StatusInternalServerError, "handover_build_failed", err.Error(), nil)
+		return
+	}
+
+	signed, err := s.Signer.Sign(ctx, s.SigningKeyRef, stmt)
+	if err != nil {
+		logger.Error("signer.Sign failed", "decision_id", req.DecisionID, "err", err)
+		if errors.Is(err, signer.ErrSigningDenied) {
+			writeErr(w, http.StatusForbidden, "signing_denied", err.Error(), nil)
+			return
+		}
+		if errors.Is(err, signer.ErrSignerUnavailable) {
+			writeErr(w, http.StatusBadGateway, "signer_unavailable", err.Error(), nil)
+			return
+		}
+		writeErr(w, http.StatusInternalServerError, "sign_failed", err.Error(), nil)
+		return
+	}
+
+	receipt, err := s.TLog.Submit(ctx, signed.Envelope, signed.ContentHash)
+	if err != nil {
+		logger.Error("tlog.Submit failed", "decision_id", req.DecisionID, "err", err)
+		writeErr(w, http.StatusBadGateway, "tlog_failed", err.Error(), nil)
+		return
+	}
+
+	writeJSON(w, http.StatusCreated, deliverResponse{
+		HandoverRef: signed.ContentHash,
+		LogIndex:    receipt.LogIndex,
+		DecisionID:  req.DecisionID,
+	})
+}
+
+func validateRequest(req deliverRequest) error {
+	if strings.TrimSpace(req.DecisionID) == "" {
+		return errors.New("decision_id is required")
+	}
+	if strings.TrimSpace(req.PayloadHash) == "" {
+		return errors.New("payload_hash is required")
+	}
+	if !strings.HasPrefix(req.PayloadHash, "sha256:") || len(req.PayloadHash) != 7+64 {
+		return errors.New("payload_hash must be sha256:<64-hex>")
+	}
+	if len(req.Payload) == 0 {
+		return errors.New("payload is required")
+	}
+	if strings.TrimSpace(req.ApprovalRef) == "" {
+		return errors.New("approval_ref is required")
+	}
+	if strings.TrimSpace(req.TransformRef) == "" {
+		return errors.New("transform_ref is required")
+	}
+	return nil
+}
+
+func writeErr(w http.ResponseWriter, status int, code, msg string, reasons []string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(errResponse{Error: errBody{
+		Code: code, Message: msg, Reasons: reasons,
+	}})
+}
+
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(v)
+}
+
+func (s Server) logger() *slog.Logger {
+	if s.Logger != nil {
+		return s.Logger
+	}
+	return slog.Default()
+}
+
+func (s Server) now() time.Time {
+	if s.Now != nil {
+		return s.Now()
+	}
+	return time.Now()
+}
diff --git a/services/dummy-sink/internal/httpapi/server_test.go b/services/dummy-sink/internal/httpapi/server_test.go
new file mode 100644
index 0000000..783e8da
--- /dev/null
+++ b/services/dummy-sink/internal/httpapi/server_test.go
@@ -0,0 +1,407 @@
+package httpapi
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/dsse"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/signer"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/sink"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/tlog"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/verify"
+)
+
+type fakeVerifier struct {
+	calls   int
+	results []verify.Result
+	lastReq verify.Request
+	err     error
+}
+
+func (f *fakeVerifier) Verify(_ context.Context, req verify.Request) (verify.Result, error) {
+	f.lastReq = req
+	if f.err != nil {
+		return verify.Result{}, f.err
+	}
+	idx := f.calls
+	if idx >= len(f.results) {
+		idx = len(f.results) - 1
+	}
+	f.calls++
+	return f.results[idx], nil
+}
+
+type fakeSigner struct {
+	calls   int
+	result  signer.SignedResult
+	err     error
+	lastStm signer.Statement
+	lastRef string
+}
+
+func (f *fakeSigner) Sign(_ context.Context, keyRef string, stmt signer.Statement) (signer.SignedResult, error) {
+	f.calls++
+	f.lastStm = stmt
+	f.lastRef = keyRef
+	if f.err != nil {
+		return signer.SignedResult{}, f.err
+	}
+	return f.result, nil
+}
+
+type fakeTLog struct {
+	calls   int
+	result  tlog.Receipt
+	err     error
+	lastEnv dsse.Envelope
+	lastHsh string
+}
+
+func (f *fakeTLog) Submit(_ context.Context, env dsse.Envelope, contentHash string) (tlog.Receipt, error) {
+	f.calls++
+	f.lastEnv = env
+	f.lastHsh = contentHash
+	if f.err != nil {
+		return tlog.Receipt{}, f.err
+	}
+	r := f.result
+	r.ContentHash = contentHash
+	return r, nil
+}
+
+func makeServerAndDeps(t *testing.T, verifier Verifier, signr Signer, tl TLog) (Server, string) {
+	t.Helper()
+	dir := t.TempDir()
+	return Server{
+		Verifier:      verifier,
+		Store:         sink.Store{DataDir: dir},
+		Signer:        signr,
+		TLog:          tl,
+		SinkID:        "sink:dummy-sink",
+		Actor:         "agent-dummy-sink",
+		SigningKeyRef: "trust-api/agent-dummy-sink-2026-q4",
+		Now:           func() time.Time { return time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC) },
+	}, dir
+}
+
+func startTestServer(t *testing.T, srv Server) *httptest.Server {
+	t.Helper()
+	mux := http.NewServeMux()
+	srv.Mount(mux)
+	ts := httptest.NewServer(mux)
+	t.Cleanup(ts.Close)
+	return ts
+}
+
+func deliverBody(t *testing.T, payload any, decisionID, approvalRef, transformRef string) ([]byte, string) {
+	t.Helper()
+	payloadRaw, err := json.Marshal(payload)
+	if err != nil {
+		t.Fatalf("marshal payload: %v", err)
+	}
+	sum := sha256.Sum256(payloadRaw)
+	payloadHash := "sha256:" + hex.EncodeToString(sum[:])
+	body, err := json.Marshal(map[string]any{
+		"decision_id":   decisionID,
+		"payload_hash":  payloadHash,
+		"payload":       json.RawMessage(payloadRaw),
+		"approval_ref":  approvalRef,
+		"transform_ref": transformRef,
+	})
+	if err != nil {
+		t.Fatalf("marshal body: %v", err)
+	}
+	return body, payloadHash
+}
+
+func TestDeliverHappyPath(t *testing.T) {
+	verifier := &fakeVerifier{results: []verify.Result{{Valid: true, DecisionID: "dec-1"}}}
+	signr := &fakeSigner{result: signer.SignedResult{
+		Envelope: dsse.Envelope{
+			PayloadType: "application/vnd.in-toto+json",
+			Payload:     "payload-b64",
+			Signatures:  []dsse.EnvelopeSignature{{KeyID: "k1", Sig: "sig1"}},
+		},
+		ContentHash: "sha256:dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd",
+		SignedAt:    time.Now(),
+	}}
+	tl := &fakeTLog{result: tlog.Receipt{LogIndex: 123}}
+
+	srv, dataDir := makeServerAndDeps(t, verifier, signr, tl)
+	ts := startTestServer(t, srv)
+
+	payload := map[string]any{"workflow_id": "WF-0042", "amount": 42.5, "memo": "coffee"}
+	body, payloadHash := deliverBody(t, payload, "dec-1",
+		"sha256:bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb",
+		"sha256:cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc",
+	)
+
+	resp, err := http.Post(ts.URL+"/api/v1/sink/dummy/deliver", "application/json", bytes.NewReader(body))
+	if err != nil {
+		t.Fatalf("POST: %v", err)
+	}
+	defer resp.Body.Close()
+	rawResp, _ := io.ReadAll(resp.Body)
+	if resp.StatusCode != http.StatusCreated {
+		t.Fatalf("status = %d, want 201; body=%s", resp.StatusCode, rawResp)
+	}
+	var out deliverResponse
+	if err := json.Unmarshal(rawResp, &out); err != nil {
+		t.Fatalf("decode resp: %v", err)
+	}
+	if out.HandoverRef != signr.result.ContentHash {
+		t.Fatalf("handover_ref = %q, want %q", out.HandoverRef, signr.result.ContentHash)
+	}
+	if out.LogIndex != 123 {
+		t.Fatalf("log_index = %d, want 123", out.LogIndex)
+	}
+	if out.DecisionID != "dec-1" {
+		t.Fatalf("decision_id = %q, want dec-1", out.DecisionID)
+	}
+
+	// Payload file was written atomically.
+	content, err := os.ReadFile(filepath.Join(dataDir, "dec-1.json"))
+	if err != nil {
+		t.Fatalf("read written file: %v", err)
+	}
+	var got map[string]any
+	if err := json.Unmarshal(content, &got); err != nil {
+		t.Fatalf("decode written file: %v", err)
+	}
+	if got["workflow_id"] != "WF-0042" {
+		t.Fatalf("written payload workflow_id = %v", got["workflow_id"])
+	}
+
+	// Verifier was called with the right fields.
+	if verifier.lastReq.DecisionID != "dec-1" ||
+		verifier.lastReq.ApprovalRef == "" ||
+		verifier.lastReq.TransformRef == "" ||
+		verifier.lastReq.SinkTarget != "sink:dummy-sink" ||
+		verifier.lastReq.PayloadHash != payloadHash {
+		t.Fatalf("verify req = %+v", verifier.lastReq)
+	}
+
+	// Signer was called with the handover statement.
+	if signr.calls != 1 {
+		t.Fatalf("signer.calls = %d, want 1", signr.calls)
+	}
+	if signr.lastRef != "trust-api/agent-dummy-sink-2026-q4" {
+		t.Fatalf("key_ref = %q", signr.lastRef)
+	}
+	if signr.lastStm["predicateType"] != "https://basicstack.de/attestations/handover/v1" {
+		t.Fatalf("predicateType = %v", signr.lastStm["predicateType"])
+	}
+
+	// TLog was called with the signed envelope + content_hash.
+	if tl.calls != 1 {
+		t.Fatalf("tlog.calls = %d, want 1", tl.calls)
+	}
+	if tl.lastHsh != signr.result.ContentHash {
+		t.Fatalf("tlog content_hash = %q, want %q", tl.lastHsh, signr.result.ContentHash)
+	}
+	if tl.lastEnv.Payload != "payload-b64" {
+		t.Fatalf("tlog envelope payload = %q", tl.lastEnv.Payload)
+	}
+}
+
+func TestDeliverInvalidChainReturns403NoWriteNoSign(t *testing.T) {
+	verifier := &fakeVerifier{results: []verify.Result{{
+		Valid:   false,
+		Reasons: []string{"approval_signature_invalid", "sink_not_allowed"},
+	}}}
+	signr := &fakeSigner{}
+	tl := &fakeTLog{}
+
+	srv, dataDir := makeServerAndDeps(t, verifier, signr, tl)
+	ts := startTestServer(t, srv)
+
+	body, _ := deliverBody(t, map[string]any{"x": 1}, "dec-bad",
+		"sha256:bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb",
+		"sha256:cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc",
+	)
+	resp, err := http.Post(ts.URL+"/api/v1/sink/dummy/deliver", "application/json", bytes.NewReader(body))
+	if err != nil {
+		t.Fatalf("POST: %v", err)
+	}
+	defer resp.Body.Close()
+	raw, _ := io.ReadAll(resp.Body)
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("status = %d, want 403; body=%s", resp.StatusCode, raw)
+	}
+	var out errResponse
+	if err := json.Unmarshal(raw, &out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(out.Error.Reasons) != 2 || out.Error.Reasons[0] != "approval_signature_invalid" {
+		t.Fatalf("reasons = %v", out.Error.Reasons)
+	}
+
+	// No file written.
+	if _, err := os.Stat(filepath.Join(dataDir, "dec-bad.json")); !os.IsNotExist(err) {
+		t.Fatalf("expected no file, stat err = %v", err)
+	}
+	if signr.calls != 0 {
+		t.Fatalf("signer must not be called; got %d", signr.calls)
+	}
+	if tl.calls != 0 {
+		t.Fatalf("tlog must not be called; got %d", tl.calls)
+	}
+}
+
+func TestDeliverReplayReturns409(t *testing.T) {
+	// First call succeeds; second call is marked as replay by the
+	// verification-service and must map to 409 with no fresh file or
+	// signing call.
+	envelope := dsse.Envelope{
+		PayloadType: "application/vnd.in-toto+json",
+		Payload:     "p1",
+		Signatures:  []dsse.EnvelopeSignature{{KeyID: "k1", Sig: "sig"}},
+	}
+	verifier := &fakeVerifier{results: []verify.Result{
+		{Valid: true, DecisionID: "dec-replay"},
+		{Valid: false, Replay: true, DecisionID: "dec-replay", Reasons: []string{"decision_id already consumed"}},
+	}}
+	signr := &fakeSigner{result: signer.SignedResult{
+		Envelope:    envelope,
+		ContentHash: "sha256:eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee",
+	}}
+	tl := &fakeTLog{result: tlog.Receipt{LogIndex: 7}}
+
+	srv, dataDir := makeServerAndDeps(t, verifier, signr, tl)
+	ts := startTestServer(t, srv)
+
+	body, _ := deliverBody(t, map[string]any{"x": "once"}, "dec-replay",
+		"sha256:bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb",
+		"sha256:cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc",
+	)
+
+	resp1, err := http.Post(ts.URL+"/api/v1/sink/dummy/deliver", "application/json", bytes.NewReader(body))
+	if err != nil {
+		t.Fatalf("POST1: %v", err)
+	}
+	resp1.Body.Close()
+	if resp1.StatusCode != http.StatusCreated {
+		t.Fatalf("first call status = %d, want 201", resp1.StatusCode)
+	}
+	if _, err := os.Stat(filepath.Join(dataDir, "dec-replay.json")); err != nil {
+		t.Fatalf("expected file after first call: %v", err)
+	}
+
+	callsAfterFirst := signr.calls
+
+	resp2, err := http.Post(ts.URL+"/api/v1/sink/dummy/deliver", "application/json", bytes.NewReader(body))
+	if err != nil {
+		t.Fatalf("POST2: %v", err)
+	}
+	defer resp2.Body.Close()
+	rawResp, _ := io.ReadAll(resp2.Body)
+	if resp2.StatusCode != http.StatusConflict {
+		t.Fatalf("second call status = %d, want 409; body=%s", resp2.StatusCode, rawResp)
+	}
+	if signr.calls != callsAfterFirst {
+		t.Fatalf("signer must not be called on replay; calls = %d (was %d)", signr.calls, callsAfterFirst)
+	}
+}
+
+func TestDeliverPayloadHashMismatch(t *testing.T) {
+	verifier := &fakeVerifier{results: []verify.Result{{Valid: true}}}
+	signr := &fakeSigner{}
+	tl := &fakeTLog{}
+	srv, _ := makeServerAndDeps(t, verifier, signr, tl)
+	ts := startTestServer(t, srv)
+
+	payload, _ := json.Marshal(map[string]any{"x": 1})
+	bogus := "sha256:" + strings.Repeat("0", 64)
+	body, _ := json.Marshal(map[string]any{
+		"decision_id":   "dec-bad-hash",
+		"payload_hash":  bogus,
+		"payload":       json.RawMessage(payload),
+		"approval_ref":  "sha256:bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb",
+		"transform_ref": "sha256:cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc",
+	})
+	resp, err := http.Post(ts.URL+"/api/v1/sink/dummy/deliver", "application/json", bytes.NewReader(body))
+	if err != nil {
+		t.Fatalf("POST: %v", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusBadRequest {
+		t.Fatalf("status = %d, want 400", resp.StatusCode)
+	}
+	if verifier.calls != 0 {
+		t.Fatalf("verify must not be called on bad hash; calls=%d", verifier.calls)
+	}
+}
+
+func TestDeliverInvalidBody(t *testing.T) {
+	verifier := &fakeVerifier{results: []verify.Result{{Valid: true}}}
+	srv, _ := makeServerAndDeps(t, verifier, &fakeSigner{}, &fakeTLog{})
+	ts := startTestServer(t, srv)
+
+	for _, body := range []string{
+		`{}`,
+		`{"decision_id":"d","payload_hash":"nothex","payload":{"x":1},"approval_ref":"a","transform_ref":"t"}`,
+		`{"decision_id":"d","payload_hash":"sha256:` + strings.Repeat("0", 64) + `","approval_ref":"a","transform_ref":"t"}`,
+	} {
+		resp, err := http.Post(ts.URL+"/api/v1/sink/dummy/deliver", "application/json", bytes.NewReader([]byte(body)))
+		if err != nil {
+			t.Fatalf("POST: %v", err)
+		}
+		resp.Body.Close()
+		if resp.StatusCode != http.StatusBadRequest {
+			t.Fatalf("status = %d, want 400 for body %s", resp.StatusCode, body)
+		}
+	}
+}
+
+func TestDeliverVerifierUnavailableReturns502(t *testing.T) {
+	verifier := &fakeVerifier{err: verify.ErrVerifyUnavailable}
+	srv, _ := makeServerAndDeps(t, verifier, &fakeSigner{}, &fakeTLog{})
+	ts := startTestServer(t, srv)
+
+	body, _ := deliverBody(t, map[string]any{"x": 1}, "d1",
+		"sha256:bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb",
+		"sha256:cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc",
+	)
+	resp, err := http.Post(ts.URL+"/api/v1/sink/dummy/deliver", "application/json", bytes.NewReader(body))
+	if err != nil {
+		t.Fatalf("POST: %v", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusBadGateway {
+		t.Fatalf("status = %d, want 502", resp.StatusCode)
+	}
+}
+
+func TestDeliverSigningDeniedPropagates403(t *testing.T) {
+	verifier := &fakeVerifier{results: []verify.Result{{Valid: true}}}
+	signr := &fakeSigner{err: errors.New("wrapped: " + signer.ErrSigningDenied.Error())}
+	// Ensure signer.err is the ErrSigningDenied sentinel so errors.Is matches.
+	signr.err = signer.ErrSigningDenied
+	srv, _ := makeServerAndDeps(t, verifier, signr, &fakeTLog{})
+	ts := startTestServer(t, srv)
+
+	body, _ := deliverBody(t, map[string]any{"x": 1}, "d1",
+		"sha256:bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb",
+		"sha256:cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc",
+	)
+	resp, err := http.Post(ts.URL+"/api/v1/sink/dummy/deliver", "application/json", bytes.NewReader(body))
+	if err != nil {
+		t.Fatalf("POST: %v", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusForbidden {
+		t.Fatalf("status = %d, want 403", resp.StatusCode)
+	}
+}
diff --git a/services/dummy-sink/internal/signer/client.go b/services/dummy-sink/internal/signer/client.go
new file mode 100644
index 0000000..0dc0494
--- /dev/null
+++ b/services/dummy-sink/internal/signer/client.go
@@ -0,0 +1,143 @@
+// Package signer calls the signing-service's POST /api/v1/sign/dsse per
+// ADR-0005 using the double-identity flow:
+//
+//   - Authorization: Bearer <pocket-id-access-token>  (OIDC client identity)
+//   - X-Workload-Token: <projected-sa-token>          (K8s workload identity)
+//
+// The signing-service's structured policy evaluates BOTH identities
+// against the Agent Registry and the OPA policy. See
+// services/signing-service/internal/httpapi/sign.go.
+package signer
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"time"
+
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/auth"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/dsse"
+)
+
+// WorkloadTokenHeader is the HTTP header carrying the projected
+// ServiceAccount token. Kept in sync with
+// services/signing-service/internal/token.WorkloadTokenHeader.
+const WorkloadTokenHeader = "X-Workload-Token"
+
+// Statement is the in-toto v1 Statement handed to the signing-service.
+type Statement map[string]any
+
+// SignedResult pairs the returned DSSE envelope with its content_hash.
+type SignedResult struct {
+	Envelope    dsse.Envelope `json:"envelope"`
+	ContentHash string        `json:"content_hash"`
+	SignedAt    time.Time     `json:"signed_at"`
+}
+
+// ErrSignerUnavailable is returned on connection errors or 5xx responses.
+var ErrSignerUnavailable = errors.New("signer: signing-service unavailable")
+
+// ErrSigningDenied is returned on 403 (OPA / registry mismatch). The
+// sink surfaces this to the caller — it is NOT retryable with the same
+// identities.
+var ErrSigningDenied = errors.New("signer: signing-service denied the request")
+
+// Client is the DSSE sign client.
+type Client struct {
+	BaseURL       string
+	HTTPClient    *http.Client
+	PocketIDToken auth.TokenSource // Authorization: Bearer …
+	WorkloadToken auth.TokenSource // X-Workload-Token: …
+}
+
+type signRequest struct {
+	KeyRef    string    `json:"key_ref"`
+	Statement Statement `json:"statement"`
+}
+
+// Sign calls POST /api/v1/sign/dsse. The signing-service returns a DSSE
+// envelope plus the stable content_hash of the envelope.
+func (c Client) Sign(ctx context.Context, keyRef string, stmt Statement) (SignedResult, error) {
+	if c.BaseURL == "" {
+		return SignedResult{}, errors.New("signer.Client: BaseURL is empty")
+	}
+	if c.PocketIDToken == nil {
+		return SignedResult{}, errors.New("signer.Client: PocketIDToken is required")
+	}
+	if c.WorkloadToken == nil {
+		return SignedResult{}, errors.New("signer.Client: WorkloadToken is required")
+	}
+	hc := c.HTTPClient
+	if hc == nil {
+		hc = &http.Client{Timeout: 10 * time.Second}
+	}
+	body, err := json.Marshal(signRequest{KeyRef: keyRef, Statement: stmt})
+	if err != nil {
+		return SignedResult{}, fmt.Errorf("signer: marshal request: %w", err)
+	}
+	endpoint, err := url.JoinPath(c.BaseURL, "/api/v1/sign/dsse")
+	if err != nil {
+		return SignedResult{}, fmt.Errorf("signer: build url: %w", err)
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, endpoint, bytes.NewReader(body))
+	if err != nil {
+		return SignedResult{}, err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept", "application/json")
+
+	pTok, err := c.PocketIDToken.Token(ctx)
+	if err != nil {
+		return SignedResult{}, fmt.Errorf("signer: pocket-id token: %w", err)
+	}
+	if pTok == "" {
+		return SignedResult{}, errors.New("signer: pocket-id token is empty")
+	}
+	req.Header.Set("Authorization", "Bearer "+pTok)
+
+	wTok, err := c.WorkloadToken.Token(ctx)
+	if err != nil {
+		return SignedResult{}, fmt.Errorf("signer: workload token: %w", err)
+	}
+	if wTok == "" {
+		return SignedResult{}, errors.New("signer: workload token is empty")
+	}
+	req.Header.Set(WorkloadTokenHeader, wTok)
+
+	resp, err := hc.Do(req)
+	if err != nil {
+		return SignedResult{}, fmt.Errorf("%w: %v", ErrSignerUnavailable, err)
+	}
+	defer resp.Body.Close()
+	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	switch {
+	case resp.StatusCode == http.StatusOK:
+		// fall through
+	case resp.StatusCode == http.StatusForbidden:
+		return SignedResult{}, fmt.Errorf("%w: %s", ErrSigningDenied, truncate(raw, 256))
+	case resp.StatusCode >= 500:
+		return SignedResult{}, fmt.Errorf("%w: status %d: %s", ErrSignerUnavailable, resp.StatusCode, truncate(raw, 256))
+	default:
+		return SignedResult{}, fmt.Errorf("signer: status %d: %s", resp.StatusCode, truncate(raw, 256))
+	}
+	var out SignedResult
+	if err := json.Unmarshal(raw, &out); err != nil {
+		return SignedResult{}, fmt.Errorf("signer: decode response: %w", err)
+	}
+	if out.ContentHash == "" {
+		return SignedResult{}, errors.New("signer: response missing content_hash")
+	}
+	return out, nil
+}
+
+func truncate(b []byte, n int) string {
+	if len(b) <= n {
+		return string(b)
+	}
+	return string(b[:n]) + "…"
+}
diff --git a/services/dummy-sink/internal/sink/storage.go b/services/dummy-sink/internal/sink/storage.go
new file mode 100644
index 0000000..5e71cc9
--- /dev/null
+++ b/services/dummy-sink/internal/sink/storage.go
@@ -0,0 +1,85 @@
+// Package sink persists verified payloads to local storage. It is the
+// trivial "write a file and be done" tail of the Phase-1 trust chain:
+// each delivery writes `<DataDir>/<decision_id>.json` atomically via
+// `.tmp` + rename so a crash cannot leave a half-written file.
+package sink
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+)
+
+// decisionIDPattern is deliberately strict so decision_id can safely be
+// pasted into a filesystem path. Approval- and verification-services
+// generate decision_id values from UUIDs / sha256 digests, so this covers
+// their output and nothing dangerous.
+var decisionIDPattern = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9._:-]{0,127}$`)
+
+// Store is a filesystem-backed payload store. All writes land in DataDir.
+type Store struct {
+	DataDir string
+}
+
+// WriteResult is the record of an atomic payload write.
+type WriteResult struct {
+	Path string
+}
+
+// ErrInvalidDecisionID is returned when decision_id would escape or
+// otherwise be unsafe on the filesystem.
+var ErrInvalidDecisionID = errors.New("sink: decision_id is not filesystem-safe")
+
+// Write atomically writes payload to <DataDir>/<decisionID>.json.
+// It writes to a sibling `<decisionID>.json.tmp` first and renames.
+// Repeated calls with the same (decisionID) replace the file; replay
+// protection is enforced by the verification-service per the Phase-1
+// contract.
+func (s Store) Write(decisionID string, payload any) (WriteResult, error) {
+	if s.DataDir == "" {
+		return WriteResult{}, errors.New("sink.Store: DataDir is required")
+	}
+	if !decisionIDPattern.MatchString(decisionID) {
+		return WriteResult{}, fmt.Errorf("%w: %q", ErrInvalidDecisionID, decisionID)
+	}
+	if err := os.MkdirAll(s.DataDir, 0o755); err != nil {
+		return WriteResult{}, fmt.Errorf("sink: ensure data dir: %w", err)
+	}
+
+	body, err := json.MarshalIndent(payload, "", "  ")
+	if err != nil {
+		return WriteResult{}, fmt.Errorf("sink: marshal payload: %w", err)
+	}
+	body = append(body, '\n')
+
+	final := filepath.Join(s.DataDir, decisionID+".json")
+	tmp := final + ".tmp"
+
+	// Open with O_TRUNC so a crashed previous .tmp cannot taint the write.
+	f, err := os.OpenFile(tmp, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0o640)
+	if err != nil {
+		return WriteResult{}, fmt.Errorf("sink: open tmp: %w", err)
+	}
+	if _, err := f.Write(body); err != nil {
+		_ = f.Close()
+		_ = os.Remove(tmp)
+		return WriteResult{}, fmt.Errorf("sink: write tmp: %w", err)
+	}
+	if err := f.Sync(); err != nil {
+		_ = f.Close()
+		_ = os.Remove(tmp)
+		return WriteResult{}, fmt.Errorf("sink: fsync tmp: %w", err)
+	}
+	if err := f.Close(); err != nil {
+		_ = os.Remove(tmp)
+		return WriteResult{}, fmt.Errorf("sink: close tmp: %w", err)
+	}
+	if err := os.Rename(tmp, final); err != nil {
+		_ = os.Remove(tmp)
+		return WriteResult{}, fmt.Errorf("sink: rename to final: %w", err)
+	}
+	return WriteResult{Path: final}, nil
+}
diff --git a/services/dummy-sink/internal/sink/storage_test.go b/services/dummy-sink/internal/sink/storage_test.go
new file mode 100644
index 0000000..8d2c64c
--- /dev/null
+++ b/services/dummy-sink/internal/sink/storage_test.go
@@ -0,0 +1,84 @@
+package sink
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestStoreWriteCreatesAtomicFile(t *testing.T) {
+	dir := t.TempDir()
+	s := Store{DataDir: dir}
+
+	payload := map[string]any{"workflow_id": "WF-0042", "amount": 42.5}
+	res, err := s.Write("decision-abc", payload)
+	if err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+	want := filepath.Join(dir, "decision-abc.json")
+	if res.Path != want {
+		t.Fatalf("Path = %q, want %q", res.Path, want)
+	}
+
+	raw, err := os.ReadFile(res.Path)
+	if err != nil {
+		t.Fatalf("ReadFile: %v", err)
+	}
+	var got map[string]any
+	if err := json.Unmarshal(raw, &got); err != nil {
+		t.Fatalf("json.Unmarshal: %v", err)
+	}
+	if got["workflow_id"] != "WF-0042" {
+		t.Fatalf("workflow_id = %v, want WF-0042", got["workflow_id"])
+	}
+	if got["amount"].(float64) != 42.5 {
+		t.Fatalf("amount = %v, want 42.5", got["amount"])
+	}
+
+	// No lingering .tmp.
+	entries, _ := os.ReadDir(dir)
+	for _, e := range entries {
+		if filepath.Ext(e.Name()) == ".tmp" {
+			t.Fatalf("left stray tmp file %q", e.Name())
+		}
+	}
+}
+
+func TestStoreWriteRejectsPathTraversal(t *testing.T) {
+	s := Store{DataDir: t.TempDir()}
+	for _, bad := range []string{
+		"",
+		"../etc/passwd",
+		"./x",
+		"with/slash",
+		"with:colon-but-starts-with-/",
+	} {
+		_, err := s.Write(bad, map[string]any{"a": 1})
+		if err == nil {
+			t.Fatalf("Write(%q) = nil err, want rejection", bad)
+		}
+	}
+}
+
+func TestStoreWriteOverwritesExisting(t *testing.T) {
+	dir := t.TempDir()
+	s := Store{DataDir: dir}
+
+	if _, err := s.Write("dec1", map[string]any{"v": 1}); err != nil {
+		t.Fatalf("first Write: %v", err)
+	}
+	if _, err := s.Write("dec1", map[string]any{"v": 2}); err != nil {
+		t.Fatalf("second Write: %v", err)
+	}
+
+	raw, err := os.ReadFile(filepath.Join(dir, "dec1.json"))
+	if err != nil {
+		t.Fatalf("ReadFile: %v", err)
+	}
+	var got map[string]any
+	_ = json.Unmarshal(raw, &got)
+	if got["v"].(float64) != 2 {
+		t.Fatalf("v = %v, want 2", got["v"])
+	}
+}
diff --git a/services/dummy-sink/internal/tlog/client.go b/services/dummy-sink/internal/tlog/client.go
new file mode 100644
index 0000000..c41785a
--- /dev/null
+++ b/services/dummy-sink/internal/tlog/client.go
@@ -0,0 +1,119 @@
+// Package tlog submits attestation envelopes to the transparency-log per
+// POST /api/v1/log/entries (ADR-0005). The TLog is idempotent on
+// content_hash: re-submitting the same (envelope, content_hash) returns
+// the original log_index.
+package tlog
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"time"
+
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/auth"
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/dsse"
+)
+
+// InclusionProof mirrors the server-side ADR-0005 inclusion proof.
+type InclusionProof struct {
+	TreeSize          int64    `json:"tree_size"`
+	TreeRootHash      string   `json:"tree_root_hash"`
+	SignedTreeHeadRef string   `json:"signed_tree_head_ref"`
+	Hashes            []string `json:"hashes"`
+}
+
+// Receipt is what the TLog returns on a successful submit.
+type Receipt struct {
+	LogIndex       int64          `json:"log_index"`
+	ContentHash    string         `json:"content_hash"`
+	LoggedAt       time.Time      `json:"logged_at"`
+	InclusionProof InclusionProof `json:"inclusion_proof"`
+}
+
+// ErrTLogUnavailable classifies connection errors and 5xx responses.
+var ErrTLogUnavailable = errors.New("tlog: transparency-log unavailable")
+
+// Client is the TLog submit client.
+type Client struct {
+	BaseURL     string
+	HTTPClient  *http.Client
+	TokenSource auth.TokenSource
+}
+
+type submitRequest struct {
+	Envelope    dsse.Envelope `json:"envelope"`
+	ContentHash string        `json:"content_hash"`
+}
+
+// Submit posts env + its contentHash to the TLog. Returns the receipt on
+// 200/201 (idempotent repeat / fresh insert) and ErrTLogUnavailable on
+// transport/5xx. Other non-2xx responses are reported with the raw status.
+func (c Client) Submit(ctx context.Context, env dsse.Envelope, contentHash string) (Receipt, error) {
+	if c.BaseURL == "" {
+		return Receipt{}, errors.New("tlog.Client: BaseURL is empty")
+	}
+	hc := c.HTTPClient
+	if hc == nil {
+		hc = &http.Client{Timeout: 10 * time.Second}
+	}
+	body, err := json.Marshal(submitRequest{Envelope: env, ContentHash: contentHash})
+	if err != nil {
+		return Receipt{}, fmt.Errorf("tlog: marshal submit: %w", err)
+	}
+	endpoint, err := url.JoinPath(c.BaseURL, "/api/v1/log/entries")
+	if err != nil {
+		return Receipt{}, fmt.Errorf("tlog: build url: %w", err)
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, endpoint, bytes.NewReader(body))
+	if err != nil {
+		return Receipt{}, err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept", "application/json")
+	if c.TokenSource != nil {
+		tok, err := c.TokenSource.Token(ctx)
+		if err != nil {
+			return Receipt{}, fmt.Errorf("tlog: token: %w", err)
+		}
+		if tok != "" {
+			req.Header.Set("Authorization", "Bearer "+tok)
+		}
+	}
+	resp, err := hc.Do(req)
+	if err != nil {
+		return Receipt{}, fmt.Errorf("%w: %v", ErrTLogUnavailable, err)
+	}
+	defer resp.Body.Close()
+	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	switch {
+	case resp.StatusCode == http.StatusOK, resp.StatusCode == http.StatusCreated:
+		// ok
+	case resp.StatusCode >= 500:
+		return Receipt{}, fmt.Errorf("%w: status %d: %s", ErrTLogUnavailable, resp.StatusCode, truncate(raw, 256))
+	default:
+		return Receipt{}, fmt.Errorf("tlog: status %d: %s", resp.StatusCode, truncate(raw, 256))
+	}
+	var out Receipt
+	if err := json.Unmarshal(raw, &out); err != nil {
+		return Receipt{}, fmt.Errorf("tlog: decode receipt: %w", err)
+	}
+	if out.ContentHash != contentHash {
+		return out, fmt.Errorf("tlog returned content_hash %q != sent %q", out.ContentHash, contentHash)
+	}
+	if out.LogIndex < 0 {
+		return out, fmt.Errorf("tlog returned negative log_index %d", out.LogIndex)
+	}
+	return out, nil
+}
+
+func truncate(b []byte, n int) string {
+	if len(b) <= n {
+		return string(b)
+	}
+	return string(b[:n]) + "…"
+}
diff --git a/services/dummy-sink/internal/verify/client.go b/services/dummy-sink/internal/verify/client.go
new file mode 100644
index 0000000..075ba31
--- /dev/null
+++ b/services/dummy-sink/internal/verify/client.go
@@ -0,0 +1,111 @@
+// Package verify calls the verification-service's POST
+// /api/v1/verify/chain endpoint. Replay protection for `decision_id` lives
+// in the verification-service — the sink does not keep its own replay
+// table. See the parent plan subtask 6 for the exact chain checks.
+package verify
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"time"
+
+	"basicstack.de/proofedmesh/services/dummy-sink/internal/auth"
+)
+
+// ErrVerifyUnavailable classifies connection errors and 5xx responses.
+var ErrVerifyUnavailable = errors.New("verify: verification-service unavailable")
+
+// Request is the payload posted to the verification-service.
+type Request struct {
+	DecisionID   string `json:"decision_id"`
+	PayloadHash  string `json:"payload_hash"`
+	ApprovalRef  string `json:"approval_ref"`
+	TransformRef string `json:"transform_ref"`
+	SinkTarget   string `json:"sink_target"`
+}
+
+// Result is the verification-service's structured verdict. The field set
+// matches the Phase-1 plan: valid, decision_id, actors, reasons, plus a
+// dedicated `replay` flag so the sink can map replays to 409.
+type Result struct {
+	Valid       bool     `json:"valid"`
+	Replay      bool     `json:"replay"`
+	DecisionID  string   `json:"decision_id"`
+	Actors      []string `json:"actors,omitempty"`
+	Reasons     []string `json:"reasons,omitempty"`
+	ApprovalRef string   `json:"approval_ref,omitempty"`
+}
+
+// Client calls POST /api/v1/verify/chain.
+type Client struct {
+	BaseURL     string
+	HTTPClient  *http.Client
+	TokenSource auth.TokenSource
+}
+
+// Verify asks the verification-service whether the chain (approval ->
+// transform) authorises writing payloadHash to sinkTarget.
+func (c Client) Verify(ctx context.Context, req Request) (Result, error) {
+	if c.BaseURL == "" {
+		return Result{}, errors.New("verify.Client: BaseURL is empty")
+	}
+	hc := c.HTTPClient
+	if hc == nil {
+		hc = &http.Client{Timeout: 10 * time.Second}
+	}
+	body, err := json.Marshal(req)
+	if err != nil {
+		return Result{}, fmt.Errorf("verify: marshal: %w", err)
+	}
+	endpoint, err := url.JoinPath(c.BaseURL, "/api/v1/verify/chain")
+	if err != nil {
+		return Result{}, fmt.Errorf("verify: build url: %w", err)
+	}
+	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, endpoint, bytes.NewReader(body))
+	if err != nil {
+		return Result{}, err
+	}
+	httpReq.Header.Set("Content-Type", "application/json")
+	httpReq.Header.Set("Accept", "application/json")
+	if c.TokenSource != nil {
+		tok, err := c.TokenSource.Token(ctx)
+		if err != nil {
+			return Result{}, fmt.Errorf("verify: token: %w", err)
+		}
+		if tok != "" {
+			httpReq.Header.Set("Authorization", "Bearer "+tok)
+		}
+	}
+	resp, err := hc.Do(httpReq)
+	if err != nil {
+		return Result{}, fmt.Errorf("%w: %v", ErrVerifyUnavailable, err)
+	}
+	defer resp.Body.Close()
+	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	switch {
+	case resp.StatusCode == http.StatusOK:
+		// fall through
+	case resp.StatusCode >= 500:
+		return Result{}, fmt.Errorf("%w: status %d: %s", ErrVerifyUnavailable, resp.StatusCode, truncate(raw, 256))
+	default:
+		return Result{}, fmt.Errorf("verify: status %d: %s", resp.StatusCode, truncate(raw, 256))
+	}
+	var out Result
+	if err := json.Unmarshal(raw, &out); err != nil {
+		return Result{}, fmt.Errorf("verify: decode: %w", err)
+	}
+	return out, nil
+}
+
+func truncate(b []byte, n int) string {
+	if len(b) <= n {
+		return string(b)
+	}
+	return string(b[:n]) + "…"
+}