fix: prevent Tool.define() wrapper accumulation on object-defined tools (#16952)

Co-authored-by: Claude Opus 4.6 <[email protected]>

2 files changed, 109 insertions, 1 deletions

diff --git a/packages/opencode/src/tool/tool.ts b/packages/opencode/src/tool/tool.ts
index 98fa50f8c..069c6557e 100644
--- a/packages/opencode/src/tool/tool.ts
+++ b/packages/opencode/src/tool/tool.ts
@@ -55,7 +55,7 @@ export namespace Tool {
     return {
       id,
       init: async (initCtx) => {
-        const toolInfo = init instanceof Function ? await init(initCtx) : init
+        const toolInfo = init instanceof Function ? await init(initCtx) : { ...init }
         const execute = toolInfo.execute
         toolInfo.execute = async (args, ctx) => {
           try {
diff --git a/packages/opencode/test/tool/tool-define.test.ts b/packages/opencode/test/tool/tool-define.test.ts
new file mode 100644
index 000000000..532510f68
--- /dev/null
+++ b/packages/opencode/test/tool/tool-define.test.ts
@@ -0,0 +1,108 @@
+import { describe, test, expect } from "bun:test"
+import z from "zod"
+import { Tool } from "../../src/tool/tool"
+
+const params = z.object({ input: z.string() })
+const defaultArgs = { input: "test" }
+
+function makeTool(id: string, executeFn?: () => void) {
+  return {
+    description: "test tool",
+    parameters: params,
+    async execute() {
+      executeFn?.()
+      return { title: "test", output: "ok", metadata: {} }
+    },
+  }
+}
+
+describe("Tool.define", () => {
+  test("object-defined tool does not mutate the original init object", async () => {
+    const original = makeTool("test")
+    const originalExecute = original.execute
+
+    const tool = Tool.define("test-tool", original)
+
+    await tool.init()
+    await tool.init()
+    await tool.init()
+
+    // The original object's execute should never be overwritten
+    expect(original.execute).toBe(originalExecute)
+  })
+
+  test("object-defined tool does not accumulate wrapper layers across init() calls", async () => {
+    let executeCalls = 0
+
+    const tool = Tool.define("test-tool", makeTool("test", () => executeCalls++))
+
+    // Call init() many times to simulate many agentic steps
+    for (let i = 0; i < 100; i++) {
+      await tool.init()
+    }
+
+    // Resolve the tool and call execute
+    const resolved = await tool.init()
+    executeCalls = 0
+
+    // Capture the stack trace inside execute to measure wrapper depth
+    let stackInsideExecute = ""
+    const origExec = resolved.execute
+    resolved.execute = async (args: any, ctx: any) => {
+      const result = await origExec.call(resolved, args, ctx)
+      const err = new Error()
+      stackInsideExecute = err.stack || ""
+      return result
+    }
+
+    await resolved.execute(defaultArgs, {} as any)
+    expect(executeCalls).toBe(1)
+
+    // Count how many times tool.ts appears in the stack.
+    // With the fix: 1 wrapper layer (from the most recent init()).
+    // Without the fix: 101 wrapper layers from accumulated closures.
+    const toolTsFrames = stackInsideExecute.split("\n").filter((l) => l.includes("tool.ts")).length
+    expect(toolTsFrames).toBeLessThan(5)
+  })
+
+  test("function-defined tool returns fresh objects and is unaffected", async () => {
+    const tool = Tool.define("test-fn-tool", () => Promise.resolve(makeTool("test")))
+
+    const first = await tool.init()
+    const second = await tool.init()
+
+    // Function-defined tools return distinct objects each time
+    expect(first).not.toBe(second)
+  })
+
+  test("object-defined tool returns distinct objects per init() call", async () => {
+    const tool = Tool.define("test-copy", makeTool("test"))
+
+    const first = await tool.init()
+    const second = await tool.init()
+
+    // Each init() should return a separate object so wrappers don't accumulate
+    expect(first).not.toBe(second)
+  })
+
+  test("validation still works after many init() calls", async () => {
+    const tool = Tool.define("test-validation", {
+      description: "validation test",
+      parameters: z.object({ count: z.number().int().positive() }),
+      async execute(args) {
+        return { title: "test", output: String(args.count), metadata: {} }
+      },
+    })
+
+    for (let i = 0; i < 100; i++) {
+      await tool.init()
+    }
+
+    const resolved = await tool.init()
+
+    const result = await resolved.execute({ count: 42 }, {} as any)
+    expect(result.output).toBe("42")
+
+    await expect(resolved.execute({ count: -1 }, {} as any)).rejects.toThrow("invalid arguments")
+  })
+})