diff --git a/.changeset/vivid-ibex-bark.md b/.changeset/vivid-ibex-bark.md
new file mode 100644
index 000000000..66d2eb602
--- /dev/null
+++ b/.changeset/vivid-ibex-bark.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 2621
+---
+**`GSD_AUDIT=1` now actually produces an audit trail** — the reference dispatch logger is wired onto the live command seam, so opting in yields the documented structured stderr line and the `.planning/.gsd-trace.jsonl` audit trail. Previously the seam built its dispatch hub without a logger, so it fell back to a no-op and the opt-in signal was inert with no indication why. With observability off, dispatch output is byte-for-byte unchanged. (#2620)
diff --git a/src/cjs-command-router-adapter.cts b/src/cjs-command-router-adapter.cts
index ddaed40c9..8af843ac9 100644
--- a/src/cjs-command-router-adapter.cts
+++ b/src/cjs-command-router-adapter.cts
@@ -13,6 +13,17 @@
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import commandRoutingHub = require('./command-routing-hub.cjs');
 const { createHub, ERROR_KINDS } = commandRoutingHub;
+// #2620 (ADR-0174 §6): the Hub defaults to a no-op logger and the live CLI
+// dispatch path never injected the reference DispatchLogger, so GSD_AUDIT and
+// config.audit.enabled were inert. Inject the reference logger ONLY when
+// observability is opt-in enabled; when off, inject nothing so the Hub stays
+// byte-for-byte silent (preserving the default dispatch output contract, incl.
+// --json-errors). Enabling stderr-on-error unconditionally by default is a
+// separate, blast-radius-bearing change (it adds a second stderr line to the
+// --json-errors envelope) — deferred as its own follow-up.
+// eslint-disable-next-line @typescript-eslint/no-require-imports
+import observabilityLogger = require('./observability/logger.cjs');
+const { createDefaultLogger, isAuditEnabled } = observabilityLogger;
 // Phase 2 (#1646): import ERROR_REASON so the UnknownCommand translation can
 // pass `sdk_unknown_command` as the second arg to error(), preserving the
 // JSON-error envelope contract that capability routers' tests assert on.
@@ -134,6 +145,10 @@ function routeHubCommandFamily({
   const hub = createHub({
     cjsRegistry: { [family]: registryHandlers },
     manifest: { [family]: available },
+    // #2620: wire the reference logger onto the live dispatch path (ADR-0174 §6),
+    // but only when observability is opt-in enabled — otherwise leave it unset
+    // so the Hub falls back to the no-op logger and stays byte-for-byte silent.
+    logger: isAuditEnabled() ? createDefaultLogger({ cwd }) : undefined,
   });
 
   const result = hub.dispatch({
diff --git a/src/observability/logger.cts b/src/observability/logger.cts
index 9bc9007b7..0d1ebd07d 100644
--- a/src/observability/logger.cts
+++ b/src/observability/logger.cts
@@ -42,9 +42,14 @@ function _safeStringify(value: unknown): string {
 }
 
 /**
- * Determine whether the audit file should be written to.
+ * Determine whether observability is opt-in enabled — via the GSD_AUDIT env var
+ * or config.audit.enabled. Exported (#2620) so the live dispatch seam can decide
+ * whether to inject the reference logger at all: when observability is off we
+ * inject nothing (the Hub stays byte-for-byte silent, preserving the default
+ * dispatch output contract, incl. --json-errors); when on, the caller injects
+ * createDefaultLogger and gets the stderr-on-error line + opt-in file audit.
  */
-function _isAuditEnabled(config: { audit?: { enabled?: boolean } } | undefined): boolean {
+function _isAuditEnabled(config?: { audit?: { enabled?: boolean } }): boolean {
   if (process.env['GSD_AUDIT'] === '1') return true;
   if (config && config.audit && config.audit.enabled === true) return true;
   return false;
@@ -163,4 +168,4 @@ function createDefaultLogger({ cwd = process.cwd(), config }: DefaultLoggerOptio
   };
 }
 
-export = { createDefaultLogger, createNoOpLogger };
+export = { createDefaultLogger, createNoOpLogger, isAuditEnabled: _isAuditEnabled };
diff --git a/src/phase-command-router.cts b/src/phase-command-router.cts
index 3d4e0822f..9ffecddf1 100644
--- a/src/phase-command-router.cts
+++ b/src/phase-command-router.cts
@@ -21,6 +21,12 @@ import { PHASE_SUBCOMMANDS } from './command-aliases.cjs';
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import commandRoutingHub = require('./command-routing-hub.cjs');
 const { createHub, ERROR_KINDS, makeInvalidArgs } = commandRoutingHub;
+// #2620 (ADR-0174 §6): inject the reference DispatchLogger on the live phase
+// dispatch path, but only when observability is opt-in enabled; otherwise the
+// Hub stays byte-for-byte silent via its no-op fallback.
+// eslint-disable-next-line @typescript-eslint/no-require-imports
+import observabilityLogger = require('./observability/logger.cjs');
+const { createDefaultLogger, isAuditEnabled } = observabilityLogger;
 
 // ─── Types ────────────────────────────────────────────────────────────────────
 
@@ -246,7 +252,10 @@ function routePhaseCommand({ phase, args, cwd, raw, error }: RoutePhaseCommandOp
 
   // ── Construct hub ──────────────────────────────────────────────────────────
   // #175: Hub is CJS-only — no mode param, no sdkLoader.
-  const hub = createHub({ cjsRegistry, manifest });
+  // #2620: wire the reference logger (ADR-0174 §6) only when observability is
+  // opt-in enabled; otherwise leave it unset so the Hub stays byte-for-byte
+  // silent via its no-op fallback.
+  const hub = createHub({ cjsRegistry, manifest, logger: isAuditEnabled() ? createDefaultLogger({ cwd }) : undefined });
 
   // ── Dispatch ────────────────────────────────────────────────────────────────
   const result = hub.dispatch({
diff --git a/tests/cjs-command-router-adapter.test.cjs b/tests/cjs-command-router-adapter.test.cjs
index be92d0564..7fb069806 100644
--- a/tests/cjs-command-router-adapter.test.cjs
+++ b/tests/cjs-command-router-adapter.test.cjs
@@ -629,3 +629,56 @@ describe('bug #3631 — SDK family routers forward --raw to output()', () => {
 });
   });
 }
+
+// ────────────────────────────────────────────────────────────────────────
+// Regression for #2620 — the live adapter must inject a real DispatchLogger
+// (ADR-0174 §6: "DispatchLogger is injected; the Hub emits DispatchEvent on
+// every dispatch path"). Before the fix, routeHubCommandFamily built the Hub
+// with no logger, so it fell back to createNoOpLogger and GSD_AUDIT was inert.
+// ────────────────────────────────────────────────────────────────────────
+{
+  const { describe, test } = require('node:test');
+  const assert = require('node:assert/strict');
+  const fs = require('node:fs');
+  const path = require('node:path');
+  const { routeHubCommandFamily } = require('../gsd-core/bin/lib/cjs-command-router-adapter.cjs');
+  const { createTempDir, cleanup } = require('./helpers.cjs');
+
+  describe('cjs-command-router-adapter — observability logger wiring (#2620)', () => {
+    test('a dispatch through the live adapter writes the opt-in audit trail when GSD_AUDIT=1', (t) => {
+      const tmp = createTempDir('gsd-obs-wire-');
+      const prev = process.env.GSD_AUDIT;
+      t.after(() => {
+        if (prev === undefined) delete process.env.GSD_AUDIT;
+        else process.env.GSD_AUDIT = prev;
+        cleanup(tmp);
+      });
+      process.env.GSD_AUDIT = '1';
+
+      routeHubCommandFamily({
+        family: 'unit',
+        args: ['unit', 'ok'],
+        subcommands: ['ok'],
+        handlers: { ok: () => {} },
+        unknownMessage: (subcommand, available) => `Unknown ${subcommand}. Available: ${available.join(', ')}`,
+        error: () => {},
+        cwd: tmp,
+        raw: false,
+      });
+
+      const auditPath = path.join(tmp, '.planning', '.gsd-trace.jsonl');
+      assert.ok(
+        fs.existsSync(auditPath),
+        'GSD_AUDIT=1 must produce .planning/.gsd-trace.jsonl — the adapter must build the Hub with a real DispatchLogger (ADR-0174 §6), not createNoOpLogger'
+      );
+      const lines = fs.readFileSync(auditPath, 'utf8').trim().split(/\r?\n/).filter(Boolean);
+      assert.ok(lines.length > 0, 'at least one dispatch event must be recorded');
+      assert.equal(lines.length, 1, 'exactly one dispatch event must be recorded');
+      const event = JSON.parse(lines[0]);
+      assert.equal(event.result.kind, 'ok', 'a successful dispatch is recorded with result.kind === "ok"');
+      assert.ok(typeof event.command === 'string' && event.command.length > 0, 'event carries the dispatched command');
+      assert.ok(typeof event.traceId === 'string' && event.traceId.length > 0, 'event carries a traceId');
+      assert.ok(typeof event.timestamp === 'string' && event.timestamp.length > 0, 'event carries an ISO timestamp');
+    });
+  });
+}
diff --git a/tests/phase-command-router.test.cjs b/tests/phase-command-router.test.cjs
index 674f99ef0..b04536634 100644
--- a/tests/phase-command-router.test.cjs
+++ b/tests/phase-command-router.test.cjs
@@ -564,3 +564,79 @@ describe('bug-1437 — phase.list-plans is wired in gsd-tools', () => {
 });
   });
 }
+
+// ─── 6. Observability logger wiring (#2620) ──────────────────────────────────
+//
+// The fix wires the reference DispatchLogger at BOTH live createHub() seams.
+// The sibling seam (cjs-command-router-adapter) is covered in its own file;
+// this pins the phase seam so a later refactor that drops the isAuditEnabled()
+// gate or the injection here cannot reintroduce the #2620 defect silently.
+{
+  const fs = require('node:fs');
+  const path = require('node:path');
+  const { createTempDir, cleanup } = require('./helpers.cjs');
+
+  describe('phase-command-router — observability logger wiring (#2620)', () => {
+    test('a dispatch through the phase seam writes the opt-in audit trail when GSD_AUDIT=1', (t) => {
+      const tmp = createTempDir('gsd-obs-phase-');
+      const prevAudit = process.env.GSD_AUDIT;
+      t.after(() => {
+        if (prevAudit === undefined) delete process.env.GSD_AUDIT;
+        else process.env.GSD_AUDIT = prevAudit;
+        cleanup(tmp);
+      });
+      process.env.GSD_AUDIT = '1';
+
+      const phase = makePhase({ cmdPhaseNextDecimal: () => {} });
+      routePhaseCommand({
+        phase,
+        args: ['phase', 'next-decimal', '5'],
+        cwd: tmp,
+        raw: false,
+        error: (m) => { throw new Error(m); },
+      });
+
+      const auditPath = path.join(tmp, '.planning', '.gsd-trace.jsonl');
+      assert.ok(
+        fs.existsSync(auditPath),
+        'GSD_AUDIT=1 must produce .planning/.gsd-trace.jsonl — the phase router must build the Hub with a real DispatchLogger (ADR-0174 §6), not createNoOpLogger'
+      );
+
+      const lines = fs.readFileSync(auditPath, 'utf8').trim().split(/\r?\n/).filter(Boolean);
+      assert.ok(lines.length > 0, 'at least one dispatch event must be recorded');
+      assert.equal(lines.length, 1, 'exactly one dispatch event must be recorded');
+
+      const event = JSON.parse(lines[0]);
+      assert.equal(event.result.kind, 'ok', 'a successful dispatch is recorded with result.kind === "ok"');
+      assert.ok(typeof event.command === 'string' && event.command.length > 0, 'event carries the dispatched command');
+      assert.ok(typeof event.traceId === 'string' && event.traceId.length > 0, 'event carries a traceId');
+      assert.ok(typeof event.timestamp === 'string' && event.timestamp.length > 0, 'event carries an ISO timestamp');
+    });
+
+    test('no audit trail is written when GSD_AUDIT is unset (Hub stays silent via the no-op fallback)', (t) => {
+      const tmp = createTempDir('gsd-obs-phase-off-');
+      const prevAudit = process.env.GSD_AUDIT;
+      t.after(() => {
+        if (prevAudit === undefined) delete process.env.GSD_AUDIT;
+        else process.env.GSD_AUDIT = prevAudit;
+        cleanup(tmp);
+      });
+      delete process.env.GSD_AUDIT;
+
+      const phase = makePhase({ cmdPhaseNextDecimal: () => {} });
+      routePhaseCommand({
+        phase,
+        args: ['phase', 'next-decimal', '5'],
+        cwd: tmp,
+        raw: false,
+        error: (m) => { throw new Error(m); },
+      });
+
+      assert.equal(
+        fs.existsSync(path.join(tmp, '.planning', '.gsd-trace.jsonl')),
+        false,
+        'without the opt-in signal the seam must leave no audit artifact'
+      );
+    });
+  });
+}