diff --git a/packages/client/runtime/src/client/sessions/request-inspection.ts b/packages/client/runtime/src/client/sessions/request-inspection.ts
index 49ff8d974f..499b9acab1 100644
--- a/packages/client/runtime/src/client/sessions/request-inspection.ts
+++ b/packages/client/runtime/src/client/sessions/request-inspection.ts
@@ -94,7 +94,8 @@ export interface RequestInspectionSnapshot {
 /**
  * Derive the request-centric read model from one immutable history window.
  * Compaction participates as a request purpose rather than a parallel
- * top-level collection.
+ * top-level collection. A leading resume/change header exposes its prompt but
+ * cannot project a change until the preceding header enters the window.
  * @param entries - Contiguous raw session history.
  * @returns Requests and call-time schemas derived from that history.
  */
@@ -218,6 +219,7 @@ function promptChange(
   prompt: ConversationPromptSnapshot,
   event: SessionEvent<'request/header'>,
 ): RequestPromptChange | undefined {
+  if (previous === undefined && event.data.reason !== 'initial') return
   const systemChanged = previous !== undefined && previous.system !== prompt.system
   const toolsChanged = previous !== undefined
     && JSON.stringify(previous.tools) !== JSON.stringify(prompt.tools)
diff --git a/packages/client/runtime/tests/request-inspection.spec.ts b/packages/client/runtime/tests/request-inspection.spec.ts
index f354d14654..bb7f6e1a9a 100644
--- a/packages/client/runtime/tests/request-inspection.spec.ts
+++ b/packages/client/runtime/tests/request-inspection.spec.ts
@@ -85,6 +85,56 @@ describe('inspectRequests', () => {
     expect(snapshot.callSchemas.get('call-1')?.name).toBe('read')
   })
 
+  it('does not promote a truncated resume or change header to the initial prompt', () => {
+    for (const reason of ['resume', 'change'] as const) {
+      const snapshot = inspectRequests(entriesOf([
+        at(10, 'step/start', { turn: 3, step: 1 }),
+        at(11, 'request/header', {
+          reason,
+          header: {
+            config: { provider: 'fake', model: 'model' },
+            system: 'tail-window prompt',
+          },
+        }),
+      ]))
+
+      expect(snapshot.requests[0]).toMatchObject({
+        purpose: 'assistant',
+        prompt: { system: 'tail-window prompt' },
+      })
+      expect(snapshot.requests[0]).not.toHaveProperty('promptChange')
+    }
+  })
+
+  it('classifies a prompt change once the preceding header is loaded', () => {
+    const snapshot = inspectRequests(entriesOf([
+      at(0, 'step/start', { turn: 1, step: 1 }),
+      at(1, 'request/header', {
+        reason: 'initial',
+        header: {
+          config: { provider: 'fake', model: 'model' },
+          system: 'before',
+        },
+      }),
+      at(2, 'step/start', { turn: 1, step: 2 }),
+      at(3, 'request/header', {
+        reason: 'change',
+        header: {
+          config: { provider: 'fake', model: 'model' },
+          system: 'after',
+        },
+      }),
+    ]))
+
+    expect(snapshot.requests[1]).toMatchObject({
+      promptChange: {
+        seq: 3,
+        kind: 'system',
+        previous: { system: 'before' },
+      },
+    })
+  })
+
   it('preserves a standalone compaction owner without widening assistant turns', () => {
     const snapshot = inspectRequests(entriesOf([
       at(0, 'compact/start', { turn: null }),