diff --git a/docs/cordis-catalog/services.md b/docs/cordis-catalog/services.md
index 9501a86c6b..0bd8a8c4a0 100644
--- a/docs/cordis-catalog/services.md
+++ b/docs/cordis-catalog/services.md
@@ -245,7 +245,7 @@ list(): Session[]
 fork(source: SessionForkSource, boundary?: number, childSessionId?: SessionId): Session
 ```
 
-Source: [`packages/core/session/src/index.ts:604`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:607`](../../packages/core/session/src/index.ts)
 
 ## `ctx.skills` — `SkillService`
 
diff --git a/docs/rfc/implemented/simplification/2026-07-12-simplify-session-log-representation.md b/docs/rfc/implemented/simplification/2026-07-12-simplify-session-log-representation.md
index a1dfb5eafb..dc51986e79 100644
--- a/docs/rfc/implemented/simplification/2026-07-12-simplify-session-log-representation.md
+++ b/docs/rfc/implemented/simplification/2026-07-12-simplify-session-log-representation.md
@@ -10,7 +10,7 @@ The session log maintains two representations that cost more machinery than thei
 
 The request-header subsystem implements a custom system/tool delta codec and transmission-decision layer even though its contract says deltas are an encoding optimization, not a reconstructability requirement. Retaining the initial/resume full snapshot at each loop-instance boundary, then writing a canonical full `request/header` whenever that instance's assembled header changes, preserves replay while deleting `SystemDelta`, `ToolsDelta`, round-trip fallback, and the durable `request/header-delta` variant. Codec-only vocabulary disappears with the codec, not because its individual arms were invalid.
 
-This proposal deliberately retains append and replacement `sourceEventSeqs`, crash-repair provenance, and all `SessionStartSource` variants: implemented RFCs give those fields an audit/interception role that zero current readers does not overturn.
+The implementation retains append and replacement `sourceEventSeqs`, crash-repair provenance, and all `SessionStartSource` variants because those fields have an audit/interception role that zero current readers does not overturn.
 
 ## Decision
 
diff --git a/packages/core/session/src/index.ts b/packages/core/session/src/index.ts
index 11451811c3..2850286cde 100644
--- a/packages/core/session/src/index.ts
+++ b/packages/core/session/src/index.ts
@@ -214,6 +214,9 @@ function assertSessionEventEnvelope(value: Record<string, unknown>, index: numbe
 
 /** Reject request-header vocabulary removed with the legacy delta codec. */
 function assertSupportedRequestHeader(type: string, data: unknown, location: string): void {
+  if (type === 'request/header-delta') {
+    throw new Error(`${location} uses unsupported legacy request/header-delta format`)
+  }
   if (type === 'request/header'
     && data !== null && typeof data === 'object' && !Array.isArray(data)
     && (data as Record<string, unknown>)['reason'] === 'fallback') {
diff --git a/packages/core/session/tests/request-header.spec.ts b/packages/core/session/tests/request-header.spec.ts
index 8bc2a4bf6c..da84ea239c 100644
--- a/packages/core/session/tests/request-header.spec.ts
+++ b/packages/core/session/tests/request-header.spec.ts
@@ -62,11 +62,17 @@ describe('foldRequestHeader', () => {
 })
 
 describe('legacy request-header format', () => {
-  it('rejects a v0 seed containing request/header-delta', () => {
+  it('rejects request/header-delta in seeds and untyped appends', () => {
     const legacy = [{
       type: 'request/header-delta', seq: 0, time: 1, data: { config: CONFIG },
     }] as unknown as SessionEvent[]
     expect(() => new Session(SessionId('legacy'), legacy)).toThrow(/unsupported legacy request\/header-delta/)
+
+    const session = new Session(SessionId('legacy-append-delta'))
+    const appendLegacy = session.append.bind(session) as (type: string, data: unknown) => SessionEvent
+    expect(() => appendLegacy('request/header-delta', { config: CONFIG }))
+      .toThrow(/unsupported legacy request\/header-delta/)
+    expect(session.events).toHaveLength(0)
   })
 
   it('rejects the removed fallback reason in seeds and untyped appends', () => {
diff --git a/packages/session-persistence/session-persistence/tests/persistence.spec.ts b/packages/session-persistence/session-persistence/tests/persistence.spec.ts
index 5d0ba2c607..f8ea43fd05 100644
--- a/packages/session-persistence/session-persistence/tests/persistence.spec.ts
+++ b/packages/session-persistence/session-persistence/tests/persistence.spec.ts
@@ -185,7 +185,7 @@ describe('SessionPersistence service registration', () => {
     await fiber.dispose()
   })
 
-  it('rejects a legacy header delta buffered by a pre-change live producer', async () => {
+  it('rejects a legacy header delta from a pre-change live producer', async () => {
     const ctx = new Context()
     await ctx.plugin(SessionStore)
     const fiber = await ctx.plugin(MemoryPersistence)
@@ -193,10 +193,9 @@ describe('SessionPersistence service registration', () => {
     // Model the runtime shape available to JavaScript or a hot-loaded plugin
     // compiled against the obsolete event vocabulary.
     const appendLegacy = session.append.bind(session) as (type: string, data: unknown) => SessionEvent
-    appendLegacy('request/header-delta', { config: { model: 'legacy' } })
-
-    await expect(ctx.sessions.flush(session))
-      .rejects.toThrow(/unsupported legacy request\/header-delta event at seq 0/)
+    expect(() => appendLegacy('request/header-delta', { config: { model: 'legacy' } }))
+      .toThrow(/unsupported legacy request\/header-delta format/)
+    expect(session.events).toHaveLength(0)
     await fiber.dispose()
   })