fix: strip thought_signature fields for cross-provider compatibility

Claude's extended thinking feature generates thought_signature fields
(message IDs like "msg_abc123...") in content blocks. When these are
sent to Google's Gemini API, it expects Base64-encoded bytes and
rejects Claude's format with a 400 error.

This commit adds stripThoughtSignatures() to remove these fields from
assistant message content blocks during sanitization, enabling session
histories to be shared across different providers (e.g., Claude → Gemini).

Fixes cross-provider session bug where switching from Claude-thinking
to Gemini (or vice versa) would fail with:
"Invalid value at 'thought_signature' (TYPE_BYTES), Base64 decoding failed"

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agents/pi-embedded-helpers.test.ts

@@ -13,6 +13,8 @@ import {
   sanitizeGoogleTurnOrdering,
   sanitizeSessionMessagesImages,
   sanitizeToolCallId,
+  stripThoughtSignatures,
+  validateGeminiTurns,
 } from "./pi-embedded-helpers.js";
 import {
   DEFAULT_AGENTS_FILENAME,
@@ -480,3 +482,176 @@ describe("sanitizeSessionMessagesImages", () => {
     ]);
   });
 });
+
+describe("normalizeTextForComparison", () => {
+  it("lowercases text", () => {
+    expect(normalizeTextForComparison("Hello World")).toBe("hello world");
+  });
+
+  it("trims whitespace", () => {
+    expect(normalizeTextForComparison("  hello  ")).toBe("hello");
+  });
+
+  it("collapses multiple spaces", () => {
+    expect(normalizeTextForComparison("hello    world")).toBe("hello world");
+  });
+
+  it("strips emoji", () => {
+    expect(normalizeTextForComparison("Hello 👋 World 🌍")).toBe("hello world");
+  });
+
+  it("handles mixed normalization", () => {
+    expect(normalizeTextForComparison("  Hello 👋   WORLD  🌍  ")).toBe(
+      "hello world",
+    );
+  });
+});
+
+describe("stripThoughtSignatures", () => {
+  it("returns non-array content unchanged", () => {
+    expect(stripThoughtSignatures("hello")).toBe("hello");
+    expect(stripThoughtSignatures(null)).toBe(null);
+    expect(stripThoughtSignatures(undefined)).toBe(undefined);
+    expect(stripThoughtSignatures(123)).toBe(123);
+  });
+
+  it("removes msg_-prefixed thought_signature from content blocks", () => {
+    const input = [
+      { type: "text", text: "hello", thought_signature: "msg_abc123" },
+      { type: "thinking", thinking: "test", thought_signature: "AQID" },
+    ];
+    const result = stripThoughtSignatures(input);
+
+    expect(result).toHaveLength(2);
+    expect(result[0]).toEqual({ type: "text", text: "hello" });
+    expect(result[1]).toEqual({
+      type: "thinking",
+      thinking: "test",
+      thought_signature: "AQID",
+    });
+    expect("thought_signature" in result[0]).toBe(false);
+    expect("thought_signature" in result[1]).toBe(true);
+  });
+
+  it("preserves blocks without thought_signature", () => {
+    const input = [
+      { type: "text", text: "hello" },
+      { type: "toolCall", id: "call_1", name: "read", arguments: {} },
+    ];
+    const result = stripThoughtSignatures(input);
+
+    expect(result).toEqual(input);
+  });
+
+  it("handles mixed blocks with and without thought_signature", () => {
+    const input = [
+      { type: "text", text: "hello", thought_signature: "msg_abc" },
+      { type: "toolCall", id: "call_1", name: "read", arguments: {} },
+      { type: "thinking", thinking: "hmm", thought_signature: "msg_xyz" },
+    ];
+    const result = stripThoughtSignatures(input);
+
+    expect(result).toEqual([
+      { type: "text", text: "hello" },
+      { type: "toolCall", id: "call_1", name: "read", arguments: {} },
+      { type: "thinking", thinking: "hmm" },
+    ]);
+  });
+
+  it("handles empty array", () => {
+    expect(stripThoughtSignatures([])).toEqual([]);
+  });
+
+  it("handles null/undefined blocks in array", () => {
+    const input = [null, undefined, { type: "text", text: "hello" }];
+    const result = stripThoughtSignatures(input);
+    expect(result).toEqual([null, undefined, { type: "text", text: "hello" }]);
+  });
+});
+
+describe("sanitizeSessionMessagesImages - thought_signature stripping", () => {
+  it("strips msg_-prefixed thought_signature from assistant message content blocks", async () => {
+    const input = [
+      {
+        role: "assistant",
+        content: [
+          { type: "text", text: "hello", thought_signature: "msg_abc123" },
+          {
+            type: "thinking",
+            thinking: "reasoning",
+            thought_signature: "AQID",
+          },
+        ],
+      },
+    ] satisfies AgentMessage[];
+
+    const out = await sanitizeSessionMessagesImages(input, "test");
+
+    expect(out).toHaveLength(1);
+    const content = (out[0] as { content?: unknown[] }).content;
+    expect(content).toHaveLength(2);
+    expect("thought_signature" in ((content?.[0] ?? {}) as object)).toBe(false);
+    expect((content?.[1] as { thought_signature?: unknown })?.thought_signature).toBe(
+      "AQID",
+    );
+  });
+});
+
+describe("isMessagingToolDuplicate", () => {
+  it("returns false for empty sentTexts", () => {
+    expect(isMessagingToolDuplicate("hello world", [])).toBe(false);
+  });
+
+  it("returns false for short texts", () => {
+    expect(isMessagingToolDuplicate("short", ["short"])).toBe(false);
+  });
+
+  it("detects exact duplicates", () => {
+    expect(
+      isMessagingToolDuplicate("Hello, this is a test message!", [
+        "Hello, this is a test message!",
+      ]),
+    ).toBe(true);
+  });
+
+  it("detects duplicates with different casing", () => {
+    expect(
+      isMessagingToolDuplicate("HELLO, THIS IS A TEST MESSAGE!", [
+        "hello, this is a test message!",
+      ]),
+    ).toBe(true);
+  });
+
+  it("detects duplicates with emoji variations", () => {
+    expect(
+      isMessagingToolDuplicate("Hello! 👋 This is a test message!", [
+        "Hello! This is a test message!",
+      ]),
+    ).toBe(true);
+  });
+
+  it("detects substring duplicates (LLM elaboration)", () => {
+    expect(
+      isMessagingToolDuplicate(
+        'I sent the message: "Hello, this is a test message!"',
+        ["Hello, this is a test message!"],
+      ),
+    ).toBe(true);
+  });
+
+  it("detects when sent text contains block reply (reverse substring)", () => {
+    expect(
+      isMessagingToolDuplicate("Hello, this is a test message!", [
+        'I sent the message: "Hello, this is a test message!"',
+      ]),
+    ).toBe(true);
+  });
+
+  it("returns false for non-matching texts", () => {
+    expect(
+      isMessagingToolDuplicate("This is completely different content.", [
+        "Hello, this is a test message!",
+      ]),
+    ).toBe(false);
+  });
+});

src/agents/pi-embedded-helpers.ts

@@ -22,6 +22,37 @@ import type { WorkspaceBootstrapFile } from "./workspace.js";
 
 export type EmbeddedContextFile = { path: string; content: string };
 
+// ── Cross-provider thought_signature sanitization ──────────────────────────────
+// Claude's extended thinking feature generates thought_signature fields (message IDs
+// like "msg_abc123...") in content blocks. When these are sent to Google's Gemini API,
+// it expects Base64-encoded bytes and rejects Claude's format with a 400 error.
+// This function strips thought_signature fields to enable cross-provider session sharing.
+
+type ContentBlockWithSignature = {
+  thought_signature?: unknown;
+  [key: string]: unknown;
+};
+
+/**
+ * Strips Claude-style thought_signature fields from content blocks.
+ *
+ * Gemini expects thought signatures as base64-encoded bytes, but Claude stores message ids
+ * like "msg_abc123...". We only strip "msg_*" to preserve any provider-valid signatures.
+ */
+export function stripThoughtSignatures<T>(content: T): T {
+  if (!Array.isArray(content)) return content;
+  return content.map((block) => {
+    if (!block || typeof block !== "object") return block;
+    const rec = block as ContentBlockWithSignature;
+    const signature = rec.thought_signature;
+    if (typeof signature !== "string" || !signature.startsWith("msg_")) {
+      return block;
+    }
+    const { thought_signature: _signature, ...rest } = rec;
+    return rest;
+  }) as T;
+}
+
 const MAX_BOOTSTRAP_CHARS = 4000;
 const BOOTSTRAP_HEAD_CHARS = 2800;
 const BOOTSTRAP_TAIL_CHARS = 800;
@@ -138,7 +169,9 @@ export async function sanitizeSessionMessagesImages(
       }
       const content = assistantMsg.content;
       if (Array.isArray(content)) {
-        const filteredContent = content.filter((block) => {
+        // Strip thought_signature fields to enable cross-provider session sharing
+        const strippedContent = stripThoughtSignatures(content);
+        const filteredContent = strippedContent.filter((block) => {
           if (!block || typeof block !== "object") return true;
           const rec = block as { type?: unknown; text?: unknown };
           if (rec.type !== "text" || typeof rec.text !== "string") return true;