diff --git a/snapshots/session/headless.snapshot.ts b/snapshots/session/headless.snapshot.ts
index 7b871e7280..2389beae9f 100644
--- a/snapshots/session/headless.snapshot.ts
+++ b/snapshots/session/headless.snapshot.ts
@@ -156,7 +156,7 @@ function records(log: string): JsonObject[] {
     .map(line => JSON.parse(line) as JsonObject)
 }
 
-/** Compare migrated expectations with native writer fields before any format-normalizing comparison. */
+/** Compare deterministic recorded tool output with raw writer fields; only message ids are volatile. */
 function verifyToolResultWriterParity(fixture: string, actual: string): void {
   expect(sessionHeaderVersion(fixture, 'retained tool-result input')).toBe(3)
   expect(sessionHeaderVersion(actual, 'raw tool-result writer')).toBe(SESSION_FORMAT_VERSION)
@@ -211,8 +211,8 @@ async function persistedSessions(cwd: string): Promise<SessionLog[]> {
   })
 }
 
-async function fixtureSessions(scenario: HeadlessScenario): Promise<string[]> {
-  const files = sessionFixtureNames(await readdir(scenario.dir))
+async function fixtureSessions(scenario: HeadlessScenario, selectedFiles?: readonly string[]): Promise<string[]> {
+  const files = selectedFiles ?? sessionFixtureNames(await readdir(scenario.dir))
   return Promise.all(files.map(async (file) => {
     const content = await readFile(join(scenario.dir, file), 'utf8')
     assertSessionFixtureVersion(file, content)
@@ -1063,14 +1063,23 @@ describe('headless recorded-session snapshots', () => {
       .toThrow('native tool-result messages match migration output')
   })
 
-  for (const scenario of scenarios) {
+  const runs = scenarios.flatMap((scenario): { scenario: HeadlessScenario; retainedToolInput?: string }[] => [
+    { scenario },
+    // This comparison retains its own V3 input even after the ordinary scenario records a newer generation.
+    ...mode === 'replay' && scenario.name === 'tool-call-turn'
+      ? [{ scenario, retainedToolInput: 'session.v3.jsonl' }] : [],
+  ])
+  for (const { scenario, retainedToolInput } of runs) {
     const skipped = scenario.manifest.platform === 'posix' && process.platform === 'win32'
       || scenario.manifest.platform === 'pwsh' && !hasPwsh
       || mode === 'record' && scenario.manifest.recording === 'authored'
       || mode === 'record' && scenario.manifest.sessionFormat !== undefined
     const scenarioTest = skipped ? it.skip : mode === 'replay' ? it.concurrent : it
-    scenarioTest(`${mode}s ${scenario.name} through dsh --profile headless`, async () => {
-      let fixtures = await fixtureSessions(scenario)
+    const inputLabel = retainedToolInput === undefined ? '' : ' from retained V3 input'
+    scenarioTest(`${mode}s ${scenario.name}${inputLabel} through dsh --profile headless`, async () => {
+      let fixtureFiles = retainedToolInput === undefined
+        ? sessionFixtureNames(await readdir(scenario.dir)) : [retainedToolInput]
+      let fixtures = await fixtureSessions(scenario, fixtureFiles)
       const primaryFixture = fixtures[0]
       if (primaryFixture === undefined) throw new Error(`${scenario.name}: missing primary session fixture`)
       const task = taskFromSession(primaryFixture) ?? scenario.manifest.input?.task
@@ -1085,7 +1094,6 @@ describe('headless recorded-session snapshots', () => {
       const composition = ownerOf(scenario)
       const baseComposition = compositionOwners.get('default')
       if (baseComposition === undefined) throw new Error('headless corpus has no default composition')
-      let fixtureFiles = sessionFixtureNames(await readdir(scenario.dir))
       const replaying = mode !== 'record'
       const compositionPatch = join(composition.dir, replaying ? 'cordis.snapshot.yml' : 'cordis.yml')
       const patchSources = [
@@ -1167,9 +1175,8 @@ describe('headless recorded-session snapshots', () => {
           },
           inspect: async (cwd) => {
             actualLogs = await persistedSessions(cwd)
-            if (replaying && scenario.name === 'tool-call-turn') {
-              const retainedV3 = await readFile(join(scenario.dir, 'session.v3.jsonl'), 'utf8')
-              verifyToolResultWriterParity(retainedV3, actualLogs[0]!.content)
+            if (retainedToolInput !== undefined) {
+              verifyToolResultWriterParity(primaryFixture, actualLogs[0]!.content)
             }
             if (mcpDemo !== undefined) {
               const log = actualLogs[0]!.content