diff --git a/examples/acp-agent/tests/acp.snapshot.ts b/examples/acp-agent/tests/acp.snapshot.ts
index fbc6af65b4..5a22209c16 100644
--- a/examples/acp-agent/tests/acp.snapshot.ts
+++ b/examples/acp-agent/tests/acp.snapshot.ts
@@ -408,6 +408,20 @@ describe('snapshot fixtures', () => {
     })
   })
 
+  it('every pinning fixture carries exactly one request/header and no deltas', async () => {
+    // The live uniformity guard runs only in NON-pinning scenarios, so a
+    // class made of just its pinning scenario (the Code Mode classes) would
+    // otherwise accept a re-recorded pin with several headers or a mid-run
+    // header-delta — shapes the pin design cannot represent. Assert the
+    // committed pins directly.
+    for (const scenario of pinningByClass.values()) {
+      const fixture = await readFile(join(SNAPSHOTS_DIR, scenario.name, 'session.jsonl'), 'utf8')
+      const headers = normalizedHeaders(fixture, fixtureContext(fixture))
+      expect(headers.length, `${scenario.name}: a pinning fixture must carry exactly one request/header`).toBe(1)
+      expect(headerDeltaCount(fixture), `${scenario.name}: a pinning fixture must carry no request/header-delta`).toBe(0)
+    }
+  })
+
   it('committed fixtures carry request-header content ONLY in the pinning scenario', async () => {
     // The whole point of the pin: a system-prompt or tool-schema change must
     // churn exactly one committed line. A non-pinning fixture that carries the
diff --git a/packages/core/tools/src/code-mode.ts b/packages/core/tools/src/code-mode.ts
index be3b7c91af..904c65ef0e 100644
--- a/packages/core/tools/src/code-mode.ts
+++ b/packages/core/tools/src/code-mode.ts
@@ -87,17 +87,21 @@ function summarize(text: string): string {
 }
 
 /**
- * JSON-normalize one binding call's argument: a `JSON.parse(JSON.stringify(…))`
- * round-trip, so the value dispatched to the tool and the value logged on the
- * `tool/code-dispatch` event are the same JSON value by construction (the
- * runtime's structured-clone boundary is wider than JSON; the session log
- * accepts only JSON). A value that does not survive (`BigInt`, a circular
- * structure, a bare function) rejects that one call with a model-correctable
- * error. `undefined` passes through — the tool's own schema validation
- * rejects it with its usual "must be an object" feedback.
+ * JSON-normalize one binding call's argument into TWO independent parses of
+ * the same canonical text: `dispatched` goes to the tool, `logged` to the
+ * `tool/code-dispatch` event — identical by construction (the runtime's
+ * structured-clone boundary is wider than JSON; the session log accepts only
+ * JSON), and separate objects, so a tool mutating its args can neither
+ * desync the log from what was dispatched nor re-poison the append. A value
+ * that does not survive the round-trip (`undefined` — the log rejects it as
+ * event data — `BigInt`, a circular structure, a bare function) rejects that
+ * one call BEFORE dispatch with a model-correctable error: nothing ever
+ * executes unlogged.
  */
-function jsonNormalizeArgs(value: unknown): unknown {
-  if (value === undefined) return undefined
+function jsonNormalizeArgs(value: unknown): { dispatched: unknown; logged: unknown } {
+  if (value === undefined) {
+    throw new Error('tool arguments must be JSON-serializable (call the tool with an arguments object, e.g. `{}`)')
+  }
   let text: string | undefined
   try {
     text = JSON.stringify(value)
@@ -108,7 +112,7 @@ function jsonNormalizeArgs(value: unknown): unknown {
   // root really yields `undefined` at runtime — the guard is live.
   // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
   if (text === undefined) throw new Error('tool arguments must be JSON-serializable (got a value JSON cannot represent)')
-  return JSON.parse(text) as unknown
+  return { dispatched: JSON.parse(text) as unknown, logged: JSON.parse(text) as unknown }
 }
 
 /** Render the program's completion value for the model-facing result text (`''` when the program returned nothing). */
@@ -198,7 +202,7 @@ export function createRunCodeTool(registry: ToolRegistry, requireRuntime: () =>
           const result = await registry.execute({
             callId: subCallId,
             name,
-            arguments: normalized,
+            arguments: normalized.dispatched,
             ...exec.agent ? { agent: exec.agent } : {},
             signal: runController.signal,
           })
@@ -212,7 +216,10 @@ export function createRunCodeTool(registry: ToolRegistry, requireRuntime: () =>
             parentCallId: exec.callId,
             subCallId,
             name,
-            arguments: normalized,
+            // The SIBLING parse of the dispatched value: byte-identical JSON,
+            // but a separate object — a tool mutating its args cannot desync
+            // this record from what it actually received.
+            arguments: normalized.logged,
             isError: result.isError,
             resultSummary: summarize(text),
           })
@@ -231,10 +238,15 @@ export function createRunCodeTool(registry: ToolRegistry, requireRuntime: () =>
         return outcome.text
       }
 
-      const functions: Record<string, CodeBindingFunction> = {}
+      // Null-prototype + defineProperty, mirroring the worker-side namespace
+      // build: a registered tool named `__proto__` must become an ordinary
+      // own key (a plain-object assignment would hit the prototype setter,
+      // silently dropping the binding), and the runtime host resolves
+      // binding names as own properties only.
+      const functions: Record<string, CodeBindingFunction> = Object.create(null) as Record<string, CodeBindingFunction>
       for (const schema of registry.schemas()) {
         if (schema.name === RUN_CODE_NAME) continue
-        functions[schema.name] = binding(schema.name)
+        Object.defineProperty(functions, schema.name, { enumerable: true, value: binding(schema.name) })
       }
 
       try {
diff --git a/packages/core/tools/tests/code-mode.spec.ts b/packages/core/tools/tests/code-mode.spec.ts
index 32dc161a86..7fcd0890e6 100644
--- a/packages/core/tools/tests/code-mode.spec.ts
+++ b/packages/core/tools/tests/code-mode.spec.ts
@@ -431,17 +431,18 @@ describe('the run_code dispatch bridge', () => {
     expect(dispatch.resultSummary.endsWith('…')).toBe(true)
   })
 
-  it('rejects undefined, JSON-throwing, and JSON-unrepresentable binding arguments', async () => {
+  it('rejects undefined, JSON-throwing, and JSON-unrepresentable binding arguments BEFORE dispatch', async () => {
     const { ctx, runtime } = await setup({ mode: 'code' })
-    registerEcho(ctx)
+    const calls = registerEcho(ctx)
+    const { agent, events } = fakeAgent()
     runtime.behavior = async (request) => {
       const echo = request.bindings[0]!.functions.echo!
       const catchMessage = (promise: Promise<unknown>) => promise.then(() => 'resolved', (error: unknown) => error instanceof Error ? error.message : String(error))
       return {
         logs: [],
         value: [
-          // undefined passes normalization untouched; the tool's own schema
-          // validation rejects it with its usual feedback.
+          // Root undefined must reject up front: the event log rejects it as
+          // data, and nothing may execute unlogged.
           await catchMessage(echo(undefined)),
           // A toJSON that throws a NON-Error propagates out of JSON.stringify.
           await catchMessage(echo({ toJSON() { throw 'raw-throw' } })),
@@ -450,11 +451,55 @@ describe('the run_code dispatch bridge', () => {
         ].join(' | '),
       }
     }
-    const result = await runCode(ctx, 'program')
+    const result = await runCode(ctx, 'program', { agent })
     const text = (result.content[0] as { text: string }).text
-    expect(text).toContain('must be an object')
+    expect(text).toContain('call the tool with an arguments object')
     expect(text).toContain('JSON-serializable: raw-throw')
     expect(text).toContain('a value JSON cannot represent')
+    // None of the three dispatched, none logged.
+    expect(calls).toEqual([])
+    expect(events.filter(event => event.type === 'tool/code-dispatch')).toEqual([])
+  })
+
+  it('logs the value the tool RECEIVED even when the tool mutates its arguments', async () => {
+    const { ctx, runtime } = await setup({ mode: 'code' })
+    const { agent, events } = fakeAgent()
+    ctx.tools.register(defineTool({
+      name: 'mutator',
+      description: 'Mutates its own args object.',
+      parameters: { list: { type: 'array', required: true } },
+      execute(args) {
+        args.list.push('injected-by-tool')
+        return Promise.resolve([{ type: 'text' as const, text: 'mutated' }])
+      },
+    }))
+    runtime.behavior = async (request) => {
+      await request.bindings[0]!.functions.mutator!({ list: ['original'] })
+      return { logs: [] }
+    }
+    const result = await runCode(ctx, 'program', { agent })
+    expect(result.isError).toBe(false)
+    const dispatch = events.find(event => event.type === 'tool/code-dispatch')?.data as SessionEventMap['tool/code-dispatch']
+    expect(dispatch.arguments).toEqual({ list: ['original'] })
+  })
+
+  it('exposes a tool named __proto__ as an ordinary own binding', async () => {
+    const { ctx, runtime } = await setup({ mode: 'code' })
+    ctx.tools.register(defineTool({
+      name: '__proto__',
+      description: 'A prototype-colliding tool name.',
+      parameters: {},
+      execute() { return Promise.resolve([{ type: 'text' as const, text: 'proto-tool-ok' }]) },
+    }))
+    runtime.behavior = async (request) => {
+      const functions = request.bindings[0]!.functions
+      expect(Object.getPrototypeOf(functions)).toBeNull()
+      const value = await functions['__proto__']!({})
+      return { logs: [], value }
+    }
+    const result = await runCode(ctx, 'program')
+    expect(result.isError).toBe(false)
+    expect(result.content[0]).toEqual({ type: 'text', text: 'proto-tool-ok' })
   })
 
   it('renders a non-string completion value inspect-style', async () => {