Merge remote-tracking branch 'origin/master' into codex/skill-system

# Conflicts:
#	docs/rfc/INDEX.md
#	examples/acp-agent/tests/snapshots/both-mode-turn/session.jsonl
#	examples/acp-agent/tests/snapshots/code-mode-turn/session.jsonl
#	examples/acp-agent/tests/snapshots/text-turn/session.jsonl
#	packages/README.md

packages/support/acp-snapshot/src/harness.ts

@@ -85,6 +85,8 @@ export type InputStep =
   | { op: 'promptExpectError'; text: string }
   | { op: 'promptAndCancel'; text: string }
   | { op: 'cancel' }
+  | { op: 'setConfigOption'; configId: string; value: string }
+  | { op: 'setConfigOptionExpectError'; configId: string; value: string }
 
 /** A scenario's `input.json`: an ordered list of input steps. */
 export interface InputScript {
@@ -408,6 +410,24 @@ async function runStep(
       await client.cancel({ sessionId })
       return
     }
+    case 'setConfigOption': {
+      const sessionId = getSessionId()
+      if (sessionId === undefined) throw new Error('snapshot-harness: setConfigOption before newSession')
+      await client.setSessionConfigOption({ sessionId, configId: step.configId, value: step.value })
+      return
+    }
+    case 'setConfigOptionExpectError': {
+      const sessionId = getSessionId()
+      if (sessionId === undefined) throw new Error('snapshot-harness: setConfigOptionExpectError before newSession')
+      // The bridge rejects an unknown id / out-of-vocabulary value; the SDK
+      // surfaces that as a rejected RPC — swallow it so the run completes and
+      // the error frame is captured in the transcript.
+      await client.setSessionConfigOption({ sessionId, configId: step.configId, value: step.value }).then(
+        () => { throw new Error('snapshot-harness: expected set_config_option to be rejected but it succeeded') },
+        () => { /* expected: the bridge rejected the id or value */ },
+      )
+      return
+    }
     default:
       throw new Error(`snapshot-harness: unknown input op ${JSON.stringify(step)}`)
   }

packages/support/acp-snapshot/src/suite.ts

@@ -98,6 +98,15 @@ export interface Scenario {
    * Defaults to false.
    */
   pinsHeader?: boolean
+  /**
+   * How many `request/header-delta` events this PINNING scenario's fixture
+   * legitimately carries (default 0). A recorded mid-run header change — a
+   * config-option switch rewriting a prompt section — is part of the pinned
+   * surface, committed verbatim like the header itself; any OTHER count
+   * still fails, so fixture rot stays caught. Meaningless off the pin (the
+   * live uniformity guard keeps non-pinning scenarios delta-free).
+   */
+  expectedHeaderDeltas?: number
   /**
    * Which header-composition class this scenario belongs to. Scenarios that
    * boot the same config compose the same header; each class has exactly one
@@ -511,17 +520,19 @@ export function defineAcpSnapshotSuite(options: SnapshotSuiteOptions): void {
       }
     })
 
-    it('every pinning fixture carries exactly one request/header and no deltas', async () => {
+    it('every pinning fixture carries exactly one request/header and its declared deltas', async () => {
       // The live uniformity guard runs only in NON-pinning scenarios, so a
       // class made of just its pinning scenario would otherwise accept a
-      // re-recorded pin with several headers or a mid-run header-delta —
-      // shapes the pin design cannot represent. Assert the committed pins
-      // directly.
+      // re-recorded pin with several headers or an undeclared mid-run
+      // header-delta — shapes the pin design cannot represent. Assert the
+      // committed pins directly; a scenario whose arc legitimately rewrites
+      // a prompt section declares the exact count via expectedHeaderDeltas.
       for (const scenario of pinningByClass.values()) {
         const fixture = await readFile(join(snapshotsDir, scenario.name, 'session.jsonl'), 'utf8')
         const headers = normalizedHeaders(fixture, fixtureContext(fixture))
         expect(headers.length, `${scenario.name}: a pinning fixture must carry exactly one request/header`).toBe(1)
-        expect(headerDeltaCount(fixture), `${scenario.name}: a pinning fixture must carry no request/header-delta`).toBe(0)
+        expect(headerDeltaCount(fixture), `${scenario.name}: a pinning fixture must carry exactly its declared request/header-deltas`)
+          .toBe(scenario.expectedHeaderDeltas ?? 0)
       }
     })