feat: auto-recover from Anthropic assistant message prefill errors

When Anthropic models reject requests with 'This model does not support
assistant message prefill', detect this as a recoverable error type and
automatically send 'Continue' once to resume the conversation.

Extends session-recovery hook with new 'assistant_prefill_unsupported'
error type. The existing session.error handler in index.ts already sends
'continue' after successful recovery, so no additional logic needed.

src/hooks/session-recovery/index.test.ts

@@ -129,6 +129,63 @@ describe("detectErrorType", () => {
     })
   })
 
+  describe("assistant_prefill_unsupported errors", () => {
+    it("should detect assistant message prefill error from direct message", () => {
+      //#given an error about assistant message prefill not being supported
+      const error = {
+        message: "This model does not support assistant message prefill. The conversation must end with a user message.",
+      }
+
+      //#when detectErrorType is called
+      const result = detectErrorType(error)
+
+      //#then should return assistant_prefill_unsupported
+      expect(result).toBe("assistant_prefill_unsupported")
+    })
+
+    it("should detect assistant message prefill error from nested error object", () => {
+      //#given an Anthropic API error with nested structure matching the real error format
+      const error = {
+        error: {
+          type: "invalid_request_error",
+          message: "This model does not support assistant message prefill. The conversation must end with a user message.",
+        },
+      }
+
+      //#when detectErrorType is called
+      const result = detectErrorType(error)
+
+      //#then should return assistant_prefill_unsupported
+      expect(result).toBe("assistant_prefill_unsupported")
+    })
+
+    it("should detect error with only 'conversation must end with a user message' fragment", () => {
+      //#given an error containing only the user message requirement
+      const error = {
+        message: "The conversation must end with a user message.",
+      }
+
+      //#when detectErrorType is called
+      const result = detectErrorType(error)
+
+      //#then should return assistant_prefill_unsupported
+      expect(result).toBe("assistant_prefill_unsupported")
+    })
+
+    it("should detect error with only 'assistant message prefill' fragment", () => {
+      //#given an error containing only the prefill mention
+      const error = {
+        message: "This model does not support assistant message prefill.",
+      }
+
+      //#when detectErrorType is called
+      const result = detectErrorType(error)
+
+      //#then should return assistant_prefill_unsupported
+      expect(result).toBe("assistant_prefill_unsupported")
+    })
+  })
+
   describe("unrecognized errors", () => {
     it("should return null for unrecognized error patterns", () => {
       // given an unrelated error

src/hooks/session-recovery/index.ts

@@ -28,6 +28,7 @@ type RecoveryErrorType =
   | "tool_result_missing"
   | "thinking_block_order"
   | "thinking_disabled_violation"
+  | "assistant_prefill_unsupported"
   | null
 
 interface MessageInfo {
@@ -126,6 +127,13 @@ function extractMessageIndex(error: unknown): number | null {
 export function detectErrorType(error: unknown): RecoveryErrorType {
   const message = getErrorMessage(error)
 
+  if (
+    message.includes("assistant message prefill") ||
+    message.includes("conversation must end with a user message")
+  ) {
+    return "assistant_prefill_unsupported"
+  }
+
   // IMPORTANT: Check thinking_block_order BEFORE tool_result_missing
   // because Anthropic's extended thinking error messages contain "tool_use" and "tool_result"
   // in the documentation URL, which would incorrectly match tool_result_missing
@@ -375,11 +383,13 @@ export function createSessionRecoveryHook(ctx: PluginInput, options?: SessionRec
         tool_result_missing: "Tool Crash Recovery",
         thinking_block_order: "Thinking Block Recovery",
         thinking_disabled_violation: "Thinking Strip Recovery",
+        assistant_prefill_unsupported: "Prefill Error Recovery",
       }
       const toastMessages: Record<RecoveryErrorType & string, string> = {
         tool_result_missing: "Injecting cancelled tool results...",
         thinking_block_order: "Fixing message structure...",
         thinking_disabled_violation: "Stripping thinking blocks...",
+        assistant_prefill_unsupported: "Sending 'Continue' to recover...",
       }
 
       await ctx.client.tui
@@ -411,6 +421,8 @@ export function createSessionRecoveryHook(ctx: PluginInput, options?: SessionRec
           const resumeConfig = extractResumeConfig(lastUser, sessionID)
           await resumeSession(ctx.client, resumeConfig)
         }
+      } else if (errorType === "assistant_prefill_unsupported") {
+        success = true
       }
 
       return success