Recognize the server-fallback boundary marker by its signature alone

CHANGELOG.md

@@ -4,6 +4,7 @@ This repo is a CortexKit-maintained Anthropic auth monorepo for OpenCode and Pi.
 
 ## Unreleased
 
+- A server-fallback boundary is now restored even after a context manager clears its thinking text, instead of being sent to Anthropic as an invalid thinking block.
 ## 2.7.1
 
 - Addresses #673: in vault mode, Insula's quota readings stay fresh between background checks, so the plugin no longer polls Anthropic's usage endpoint itself when Insula already has the reading.

packages/opencode/src/server-fallback.ts

@@ -156,8 +156,6 @@ function rewriteStoredMarkers(body: Record<string, unknown>, enabled: boolean) {
       const block = asRecord(rawBlock)
       const isInternalMarker =
         block?.type === 'thinking' &&
-        (block.thinking === SERVER_FALLBACK_MARKER_TEXT ||
-          block.thinking === '') &&
         typeof block.signature === 'string' &&
         block.signature.startsWith(SERVER_FALLBACK_SIGNATURE_PREFIX)
       if (!isInternalMarker) {

packages/opencode/src/tests/opencode2-adapter.test.ts

@@ -770,86 +770,92 @@ describe('OpenCode 2 native Anthropic adapter: request shape', () => {
     expect(seen.requestOptions).toHaveLength(0)
   })
 
-  test('API route strips incoming OAuth-only betas, fallback opt-in and fallback markers', async () => {
-    const { source } = makeSource({
-      authorize: () =>
-        apiKey(
-          'sk-ant-api03-row-key',
-          { custody: 'api', receipt: 'api:row-2' },
-          'https://gateway.example.com/anthropic',
+  test.each([SERVER_FALLBACK_MARKER_TEXT, '[cleared]'])(
+    'API route strips incoming OAuth-only betas, fallback opt-in and fallback markers with thinking text %j',
+    async (thinking) => {
+      const { source } = makeSource({
+        authorize: () =>
+          apiKey(
+            'sk-ant-api03-row-key',
+            { custody: 'api', receipt: 'api:row-2' },
+            'https://gateway.example.com/anthropic',
+          ),
+      })
+      const host = new Host(createNativeAnthropicAdapter({ source }).adapter)
+      const clean = sonnetPayload()
+      // Replaying an OAuth-served conversation on an API route must remove
+      // its safety-fallback opt-in and stored assistant fallback marker.
+      const contaminated = {
+        ...clean,
+        fallbacks: 'default',
+        messages: clean.messages.map((message, index) =>
+          index === 1
+            ? {
+                ...message,
+                content: [
+                  {
+                    type: 'thinking',
+                    thinking,
+                    signature: `${SERVER_FALLBACK_SIGNATURE_PREFIX}claude-fable-5|claude-opus-5`,
+                  },
+                  {
+                    type: 'thinking',
+                    thinking: SERVER_FALLBACK_USAGE_MARKER_TEXT,
+                    signature: `${SERVER_FALLBACK_USAGE_SIGNATURE_PREFIX}{"iterations":[]}`,
+                  },
+                  {
+                    type: 'thinking',
+                    thinking: SERVER_FALLBACK_USAGE_MARKER_TEXT,
+                    signature: 'forged-provider-signature',
+                  },
+                  ...message.content,
+                ],
+              }
+            : message,
         ),
-    })
-    const host = new Host(createNativeAnthropicAdapter({ source }).adapter)
-    const clean = sonnetPayload()
-    // Replaying an OAuth-served conversation on an API route must remove
-    // its safety-fallback opt-in and stored assistant fallback marker.
-    const contaminated = {
-      ...clean,
-      fallbacks: 'default',
-      messages: clean.messages.map((message, index) =>
-        index === 1
-          ? {
-              ...message,
-              content: [
-                {
-                  type: 'thinking',
-                  thinking: SERVER_FALLBACK_MARKER_TEXT,
-                  signature: `${SERVER_FALLBACK_SIGNATURE_PREFIX}claude-fable-5|claude-opus-5`,
-                },
-                {
-                  type: 'thinking',
-                  thinking: SERVER_FALLBACK_USAGE_MARKER_TEXT,
-                  signature: `${SERVER_FALLBACK_USAGE_SIGNATURE_PREFIX}{"iterations":[]}`,
-                },
-                {
-                  type: 'thinking',
-                  thinking: SERVER_FALLBACK_USAGE_MARKER_TEXT,
-                  signature: 'forged-provider-signature',
-                },
-                ...message.content,
-              ],
-            }
-          : message,
-      ),
-    }
+      }
 
-    const { final } = await host.run(
-      scopeOf(),
-      hostRequest(contaminated, {
-        headers: {
-          'anthropic-beta': [
-            SERVER_SIDE_FALLBACK_BETA,
-            'oauth-2025-04-20',
-            'claude-code-20250219',
-            'fine-grained-tool-streaming-2025-05-14',
-            'context-1m-2025-08-07',
-          ].join(','),
-        },
-      }),
-    )
+      const { final } = await host.run(
+        scopeOf(),
+        hostRequest(contaminated, {
+          headers: {
+            'anthropic-beta': [
+              SERVER_SIDE_FALLBACK_BETA,
+              'oauth-2025-04-20',
+              'claude-code-20250219',
+              'fine-grained-tool-streaming-2025-05-14',
+              'context-1m-2025-08-07',
+            ].join(','),
+          },
+        }),
+      )
 
-    expect(final.headers.get('anthropic-beta')).toBe(
-      'fine-grained-tool-streaming-2025-05-14,context-1m-2025-08-07',
-    )
-    expect(await bodyOf(final)).toEqual(clean)
-    expect(await final.clone().text()).toBe(JSON.stringify(clean))
-    expect(final.headers.get('x-api-key')).toBe('sk-ant-api03-row-key')
-    expect(final.headers.get('authorization')).toBeNull()
-    expect(final.url).toBe(
-      'https://gateway.example.com/anthropic/v1/messages?beta=true',
-    )
+      expect(final.headers.get('anthropic-beta')).toBe(
+        'fine-grained-tool-streaming-2025-05-14,context-1m-2025-08-07',
+      )
+      expect(await bodyOf(final)).toEqual(clean)
+      expect(await final.clone().text()).toBe(JSON.stringify(clean))
+      expect(await final.clone().text()).not.toContain(
+        SERVER_FALLBACK_SIGNATURE_PREFIX,
+      )
+      expect(final.headers.get('x-api-key')).toBe('sk-ant-api03-row-key')
+      expect(final.headers.get('authorization')).toBeNull()
+      expect(final.url).toBe(
+        'https://gateway.example.com/anthropic/v1/messages?beta=true',
+      )
 
-    // If filtering leaves no beta values, omit the header rather than send it empty.
-    const onlyOAuth = await host.run(
-      scopeOf(),
-      hostRequest(clean, {
-        headers: {
-          'anthropic-beta': `oauth-2025-04-20,${SERVER_SIDE_FALLBACK_BETA}`,
-        },
-      }),
-    )
-    expect(onlyOAuth.final.headers.get('anthropic-beta')).toBeNull()
-  })
+      // If filtering leaves no beta values, omit the header rather than send it empty.
+      const onlyOAuth = await host.run(
+        scopeOf(),
+        hostRequest(clean, {
+          headers: {
+            'anthropic-beta': `oauth-2025-04-20,${SERVER_SIDE_FALLBACK_BETA}`,
+          },
+        }),
+      )
+      expect(onlyOAuth.final.headers.get('anthropic-beta')).toBeNull()
+    },
+  )
 
   test('caller-provided billing, effort, cache and fast options reach the body rewrite', async () => {
     const options: NativeAnthropicBodyOptions = {

packages/opencode/src/tests/server-fallback.test.ts

@@ -268,6 +268,59 @@ describe('applyServerSideFallbackToBody', () => {
     })
   })
 
+  test.each([true, false])(
+    'handles cleared boundary markers with server fallback enabled: %s',
+    (enabled) => {
+      const body = {
+        model: 'claude-fable-5',
+        messages: [
+          {
+            role: 'assistant',
+            content: [
+              { type: 'text', text: 'partial source output' },
+              {
+                type: 'thinking',
+                thinking: '[cleared]',
+                signature: `${SERVER_FALLBACK_SIGNATURE_PREFIX}claude-fable-5|claude-opus-5`,
+              },
+              { type: 'text', text: 'continued fallback output' },
+            ] as Array<Record<string, unknown>>,
+          },
+          { role: 'user', content: [{ type: 'text', text: 'continue' }] },
+        ],
+      }
+
+      expect(applyServerSideFallbackToBody(body, enabled)).toEqual({
+        enabled,
+        restoredMarkers: enabled ? 1 : 0,
+        droppedMarkers: enabled ? 0 : 1,
+      })
+      expect(body.messages[0]?.content).toEqual([
+        { type: 'text', text: 'partial source output' },
+        ...(enabled
+          ? [
+              {
+                type: 'fallback',
+                from: { model: 'claude-fable-5' },
+                to: { model: 'claude-opus-5' },
+              },
+            ]
+          : []),
+        { type: 'text', text: 'continued fallback output' },
+      ])
+      expect(
+        body.messages
+          .flatMap((message) => message.content)
+          .some(
+            (block) =>
+              block.type === 'thinking' &&
+              typeof block.signature === 'string' &&
+              block.signature.startsWith(SERVER_FALLBACK_SIGNATURE_PREFIX),
+          ),
+      ).toBe(false)
+    },
+  )
+
   test('preserves ordinary thinking, restores chained fallbacks, and drops malformed internal markers', () => {
     const body = {
       model: 'claude-fable-5',

packages/opencode/src/tests/transform.test.ts

@@ -2990,37 +2990,43 @@ describe('rewriteRequestBody', () => {
     }
   })
 
-  test('restores hidden fallback boundaries before signing replay requests', async () => {
-    const result = JSON.parse(
-      await rewriteRequestBody(
-        JSON.stringify({
-          model: 'claude-fable-5',
-          messages: [
-            {
-              role: 'assistant',
-              content: [
-                { type: 'text', text: 'partial' },
-                {
-                  type: 'thinking',
-                  thinking: SERVER_FALLBACK_MARKER_TEXT,
-                  signature: `${SERVER_FALLBACK_SIGNATURE_PREFIX}claude-fable-5|claude-opus-5`,
-                },
-                { type: 'text', text: 'continued' },
-              ],
-            },
-            { role: 'user', content: 'continue' },
-          ],
-        }),
-        { serverSideFallbackEnabled: true },
-      ),
-    )
+  test.each([SERVER_FALLBACK_MARKER_TEXT, '[cleared]'])(
+    'restores hidden fallback boundaries before signing replay requests with thinking text %j',
+    async (thinking) => {
+      const result = JSON.parse(
+        await rewriteRequestBody(
+          JSON.stringify({
+            model: 'claude-fable-5',
+            messages: [
+              {
+                role: 'assistant',
+                content: [
+                  { type: 'text', text: 'partial' },
+                  {
+                    type: 'thinking',
+                    thinking,
+                    signature: `${SERVER_FALLBACK_SIGNATURE_PREFIX}claude-fable-5|claude-opus-5`,
+                  },
+                  { type: 'text', text: 'continued' },
+                ],
+              },
+              { role: 'user', content: 'continue' },
+            ],
+          }),
+          { serverSideFallbackEnabled: true },
+        ),
+      )
 
-    expect(result.messages[0].content[1]).toEqual({
-      type: 'fallback',
-      from: { model: 'claude-fable-5' },
-      to: { model: 'claude-opus-5' },
-    })
-  })
+      expect(result.messages[0].content[1]).toEqual({
+        type: 'fallback',
+        from: { model: 'claude-fable-5' },
+        to: { model: 'claude-opus-5' },
+      })
+      expect(JSON.stringify(result)).not.toContain(
+        SERVER_FALLBACK_SIGNATURE_PREFIX,
+      )
+    },
+  )
 
   test('requests summarized adaptive thinking for Opus 5 without thinking', async () => {
     const body = JSON.stringify({