diff --git a/src/agents/pi-tool-definition-adapter.test.ts b/src/agents/pi-tool-definition-adapter.test.ts
index 6def07167cb..1bc8ad94db4 100644
--- a/src/agents/pi-tool-definition-adapter.test.ts
+++ b/src/agents/pi-tool-definition-adapter.test.ts
@@ -1,7 +1,8 @@
 import type { AgentTool } from "@mariozechner/pi-agent-core";
 import { Type } from "@sinclair/typebox";
 import { describe, expect, it } from "vitest";
-import { toToolDefinitions } from "./pi-tool-definition-adapter.js";
+import type { ClientToolDefinition } from "./pi-embedded-runner/run/params.js";
+import { toClientToolDefinitions, toToolDefinitions } from "./pi-tool-definition-adapter.js";
 
 type ToolExecute = ReturnType<typeof toToolDefinitions>[number]["execute"];
 const extensionContext = {} as Parameters<ToolExecute>[4];
@@ -98,3 +99,81 @@ describe("pi tool definition adapter", () => {
     expect((result.content[0] as { text?: string }).text).toContain('"count"');
   });
 });
+
+// ---------------------------------------------------------------------------
+// toClientToolDefinitions – streaming tool-call argument coercion (#57009)
+// ---------------------------------------------------------------------------
+
+function makeClientTool(name: string): ClientToolDefinition {
+  return {
+    type: "function",
+    function: {
+      name,
+      description: `${name} tool`,
+      parameters: { type: "object", properties: { query: { type: "string" } } },
+    },
+  };
+}
+
+async function executeClientTool(
+  params: unknown,
+): Promise<{ calledWith: Record<string, unknown> | undefined }> {
+  let captured: Record<string, unknown> | undefined;
+  const [def] = toClientToolDefinitions([makeClientTool("search")], (_name, p) => {
+    captured = p;
+  });
+  if (!def) {
+    throw new Error("missing client tool definition");
+  }
+  await def.execute("call-c1", params, undefined, undefined, extensionContext);
+  return { calledWith: captured };
+}
+
+describe("toClientToolDefinitions – param coercion", () => {
+  it("passes plain object params through unchanged", async () => {
+    const { calledWith } = await executeClientTool({ query: "hello" });
+    expect(calledWith).toEqual({ query: "hello" });
+  });
+
+  it("parses a JSON string into an object (streaming delta accumulation)", async () => {
+    const { calledWith } = await executeClientTool('{"query":"hello","limit":10}');
+    expect(calledWith).toEqual({ query: "hello", limit: 10 });
+  });
+
+  it("parses a JSON string with surrounding whitespace", async () => {
+    const { calledWith } = await executeClientTool('  {"query":"hello"}  ');
+    expect(calledWith).toEqual({ query: "hello" });
+  });
+
+  it("falls back to empty object for invalid JSON string", async () => {
+    const { calledWith } = await executeClientTool("not-json");
+    expect(calledWith).toEqual({});
+  });
+
+  it("falls back to empty object for empty string", async () => {
+    const { calledWith } = await executeClientTool("");
+    expect(calledWith).toEqual({});
+  });
+
+  it("falls back to empty object for null", async () => {
+    const { calledWith } = await executeClientTool(null);
+    expect(calledWith).toEqual({});
+  });
+
+  it("falls back to empty object for undefined", async () => {
+    const { calledWith } = await executeClientTool(undefined);
+    expect(calledWith).toEqual({});
+  });
+
+  it("falls back to empty object for a JSON array string", async () => {
+    const { calledWith } = await executeClientTool("[1,2,3]");
+    expect(calledWith).toEqual({});
+  });
+
+  it("handles nested JSON string correctly", async () => {
+    const { calledWith } = await executeClientTool(
+      '{"action":"search","params":{"q":"test","page":1}}',
+    );
+    expect(calledWith).toEqual({ action: "search", params: { q: "test", page: 1 } });
+  });
+});
diff --git a/src/agents/pi-tool-definition-adapter.ts b/src/agents/pi-tool-definition-adapter.ts
index d2673ff46b1..da345a74ea8 100644
--- a/src/agents/pi-tool-definition-adapter.ts
+++ b/src/agents/pi-tool-definition-adapter.ts
@@ -172,6 +172,38 @@ export function toToolDefinitions(tools: AnyAgentTool[]): ToolDefinition[] {
   });
 }
 
+/**
+ * Coerce tool-call params into a plain object.
+ *
+ * Some providers (e.g. Gemini) stream tool-call arguments as incremental
+ * string deltas.  By the time the framework invokes the tool's `execute`
+ * callback the accumulated value may still be a JSON **string** rather than
+ * a parsed object.  `isPlainObject()` returns `false` for strings, which
+ * caused the params to be silently replaced with `{}`.
+ *
+ * This helper tries `JSON.parse` when the value is a string and falls back
+ * to an empty object only when parsing genuinely fails.
+ */
+function coerceParamsRecord(value: unknown): Record<string, unknown> {
+  if (isPlainObject(value)) {
+    return value;
+  }
+  if (typeof value === "string") {
+    const trimmed = value.trim();
+    if (trimmed.length > 0) {
+      try {
+        const parsed: unknown = JSON.parse(trimmed);
+        if (isPlainObject(parsed)) {
+          return parsed;
+        }
+      } catch {
+        // not valid JSON – fall through to empty object
+      }
+    }
+  }
+  return {};
+}
+
 // Convert client tools (OpenResponses hosted tools) to ToolDefinition format
 // These tools are intercepted to return a "pending" result instead of executing
 export function toClientToolDefinitions(
@@ -198,7 +230,7 @@ export function toClientToolDefinitions(
           throw new Error(outcome.reason);
         }
         const adjustedParams = outcome.params;
-        const paramsRecord = isPlainObject(adjustedParams) ? adjustedParams : {};
+        const paramsRecord = coerceParamsRecord(adjustedParams);
         // Notify handler that a client tool was called
         if (onClientToolCall) {
           onClientToolCall(func.name, paramsRecord);