fix: add missing arguments/input field to tool calls

When models call tools without parameters, they sometimes omit the
arguments/input field entirely. Anthropic/Cloud Code Assist APIs require
this field to be present (even if empty {}), causing the error:

  messages.X.content.Y.tool_use.input: Field required

This corrupts the session history and prevents further messages.

Fix: Add sanitizeToolCallArguments() in images.ts that ensures all
toolCall, toolUse, and functionCall blocks have the required field.

Closes #issue-number

src/agents/pi-embedded-helpers.sanitize-session-messages-images.removes-empty-assistant-text-blocks-but-preserves.test.ts

@@ -117,4 +117,48 @@ describe("sanitizeSessionMessagesImages", () => {
     expect(out[0]?.role).toBe("user");
     expect(out[1]?.role).toBe("toolResult");
   });
+
+  it("adds missing arguments field to tool calls", async () => {
+    const input = [
+      {
+        role: "assistant",
+        content: [
+          { type: "toolCall", id: "call_1", name: "session_status" }, // Missing arguments!
+        ],
+      },
+    ] satisfies AgentMessage[];
+
+    const out = await sanitizeSessionMessagesImages(input, "test");
+
+    expect(out).toHaveLength(1);
+    const content = (out[0] as { content?: unknown }).content as Array<{
+      type?: string;
+      arguments?: unknown;
+    }>;
+    expect(content).toHaveLength(1);
+    expect(content[0]?.type).toBe("toolCall");
+    expect(content[0]?.arguments).toEqual({});
+  });
+
+  it("adds missing input field to toolUse blocks", async () => {
+    const input = [
+      {
+        role: "assistant",
+        content: [
+          { type: "toolUse", id: "call_1", name: "session_status" }, // Missing input!
+        ],
+      },
+    ] satisfies AgentMessage[];
+
+    const out = await sanitizeSessionMessagesImages(input, "test");
+
+    expect(out).toHaveLength(1);
+    const content = (out[0] as { content?: unknown }).content as Array<{
+      type?: string;
+      input?: unknown;
+    }>;
+    expect(content).toHaveLength(1);
+    expect(content[0]?.type).toBe("toolUse");
+    expect(content[0]?.input).toEqual({});
+  });
 });

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

@@ -21,6 +21,25 @@ export function isEmptyAssistantMessageContent(
   });
 }
 
+/**
+ * Ensures tool call blocks have the required arguments/input field.
+ * Some models omit this field when calling tools with no parameters,
+ * but Anthropic/Cloud Code Assist APIs require it (even if empty `{}`).
+ */
+function sanitizeToolCallArguments(content: unknown[]): unknown[] {
+  return content.map((block) => {
+    if (!block || typeof block !== "object") return block;
+    const rec = block as { type?: unknown; arguments?: unknown; input?: unknown };
+    if (rec.type === "toolCall" && rec.arguments === undefined) {
+      return { ...rec, arguments: {} };
+    }
+    if ((rec.type === "toolUse" || rec.type === "functionCall") && rec.input === undefined) {
+      return { ...rec, input: {} };
+    }
+    return block;
+  });
+}
+
 export async function sanitizeSessionMessagesImages(
   messages: AgentMessage[],
   label: string,
@@ -97,17 +116,19 @@ export async function sanitizeSessionMessagesImages(
       }
       const content = assistantMsg.content;
       if (Array.isArray(content)) {
+        // Ensure all tool calls have required arguments/input field
+        const sanitizedContent = sanitizeToolCallArguments(content);
         if (!allowNonImageSanitization) {
           const nextContent = (await sanitizeContentBlocksImages(
-            content as unknown as ContentBlock[],
+            sanitizedContent as unknown as ContentBlock[],
             label,
           )) as unknown as typeof assistantMsg.content;
           out.push({ ...assistantMsg, content: nextContent });
           continue;
         }
         const strippedContent = options?.preserveSignatures
-          ? content // Keep signatures for Antigravity Claude
-          : stripThoughtSignatures(content, options?.sanitizeThoughtSignatures); // Strip for Gemini
+          ? sanitizedContent // Keep signatures for Antigravity Claude
+          : stripThoughtSignatures(sanitizedContent, options?.sanitizeThoughtSignatures); // Strip for Gemini
 
         const filteredContent = strippedContent.filter((block) => {
           if (!block || typeof block !== "object") return true;