diff --git a/src/query/proxyToolCommit.test.ts b/src/query/proxyToolCommit.test.ts
index e6921471..a9551d0c 100644
--- a/src/query/proxyToolCommit.test.ts
+++ b/src/query/proxyToolCommit.test.ts
@@ -7,7 +7,7 @@ import { createSandboxedTestEnvironment } from '../../scripts/pr/test-environmen
 import type { Tool, ToolUseContext } from '../Tool.js'
 import type { QueryParams } from '../query.js'
 
-const scenarios = ['chat-eof', 'chat-length', 'chat-error', 'chat-completed', 'responses-incomplete', 'responses-failed', 'responses-done-only', 'anthropic-duplicate', 'anthropic-eof', 'anthropic-truncated'] as const
+const scenarios = ['chat-eof', 'chat-length', 'chat-error', 'chat-completed', 'responses-incomplete', 'responses-failed', 'responses-done-only', 'anthropic-duplicate', 'anthropic-eof', 'anthropic-truncated', 'chat-malformed-corrected', 'chat-malformed-repeated', 'chat-mixed-corrected'] as const
 type Scenario = typeof scenarios[number]
 const resultPrefix = 'PROXY_TOOL_COMMIT_RESULT:'
 const childScenario = process.env.CC_HAHA_PROXY_TOOL_COMMIT_SCENARIO
@@ -37,9 +37,16 @@ async function runScenario(root: string, scenario: Scenario) {
   let executions = 0
   const committedToolIds: string[] = []
   let requests = 0
+  let maxTurnsReached = false
+  const executedPaths: string[] = []
+  const feedback: Array<{ request: number; id: string; error: boolean; content: string }> = []
+  const correctionScenario = scenario === 'chat-malformed-corrected' || scenario === 'chat-mixed-corrected'
+  const repeatedMalformed = scenario === 'chat-malformed-repeated'
+  const malformedScenario = correctionScenario || repeatedMalformed
   const target = join(root, `${scenario}.txt`)
   const input = { file_path: target, content: 'written exactly once' }
   const args = JSON.stringify(input)
+  const malformedArgs = JSON.stringify({ file_path: target }).slice(0, -1) + ',"content": invalid}'
   const isChat = scenario.startsWith('chat-')
   let wire = isChat
     ? chatTool(args, scenario === 'chat-completed' ? 'tool_calls' : scenario === 'chat-length' ? 'length' : undefined)
@@ -59,8 +66,25 @@ async function runScenario(root: string, scenario: Scenario) {
   }
   if (scenario === 'chat-error') wire += `data: ${JSON.stringify({ error: { type: 'server_error', message: 'fixture upstream failure' } })}\n\n`
   const server = Bun.serve({ hostname: '127.0.0.1', port: 0, async fetch(request) {
-    await request.json()
+    const body = await request.json() as { messages?: Array<{ content?: unknown }> }
     requests++
+    for (const message of body.messages ?? []) {
+      if (!Array.isArray(message.content)) continue
+      for (const block of message.content) {
+        if (block?.type === 'tool_result') feedback.push({ request: requests, id: block.tool_use_id, error: block.is_error === true, content: JSON.stringify(block.content) })
+      }
+    }
+    if (malformedScenario) {
+      let calls: Array<{ id: string; arguments: string }> = []
+      if (requests === 1 || repeatedMalformed) {
+        calls = [{ id: repeatedMalformed ? `call_invalid_${requests}` : 'call_fixture', arguments: malformedArgs }]
+        if (scenario === 'chat-mixed-corrected') calls.unshift({ id: 'call_valid', arguments: JSON.stringify({ ...input, file_path: join(root, 'already-written.txt') }) })
+      } else if (requests === 2) calls = [{ id: 'call_corrected', arguments: args }]
+      const correctedWire = calls.length
+        ? chatTools(calls)
+        : `data: ${JSON.stringify({ choices: [{ delta: { content: 'complete' }, finish_reason: 'stop' }] })}\n\n`
+      return new Response(openaiChatStreamToAnthropic(upstream(correctedWire), 'fixture-model'), { headers: { 'content-type': 'text/event-stream' } })
+    }
     const stream = requests === 1
       ? (scenario.startsWith('anthropic-') ? upstream(wire) : isChat ? openaiChatStreamToAnthropic(upstream(wire), 'fixture-model') : openaiResponsesStreamToAnthropic(upstream(wire), 'fixture-model'))
       : openaiChatStreamToAnthropic(upstream(`data: ${JSON.stringify({ choices: [{ delta: { content: 'complete' }, finish_reason: 'stop' }] })}\n\n`), 'fixture-model')
@@ -73,6 +97,7 @@ async function runScenario(root: string, scenario: Scenario) {
     isEnabled: () => true, userFacingName: () => 'fixture write', description: async () => 'Write a fixture file',
     call: async (value: typeof input) => {
       executions++
+      executedPaths.push(value.file_path)
       await writeFile(value.file_path, value.content)
       return { data: 'written' }
     },
@@ -92,18 +117,19 @@ async function runScenario(root: string, scenario: Scenario) {
     messages: [createUserMessage({ content: 'Run the fixture write once' })],
     systemPrompt: asSystemPrompt([]), userContext: {}, systemContext: {},
     canUseTool: async (_tool, value) => ({ behavior: 'allow', updatedInput: value }),
-    toolUseContext, querySource: 'sdk', maxTurns: 2,
+    toolUseContext, querySource: 'sdk', maxTurns: malformedScenario ? 3 : 2,
     deps: { callModel, microcompact: async messages => ({ messages }), autocompact: async () => ({}), uuid: randomUUID },
   }
   try {
     for await (const message of query(params)) {
+      if (message.type === 'attachment' && message.attachment.type === 'max_turns_reached') maxTurnsReached = true
       if (message.type === 'assistant') {
         for (const block of message.message.content) {
           if (block.type === 'tool_use') committedToolIds.push(block.id)
         }
       }
     }
-    return { executions, requests, committedToolIds }
+    return { executions, requests, committedToolIds, executedPaths, feedback, maxTurnsReached }
   } finally {
     toolUseContext.abortController.abort()
     server.stop(true)
@@ -123,6 +149,12 @@ function event(type: string, fields: Record<string, unknown>): string {
   return `event: ${type}\ndata: ${JSON.stringify({ type, ...fields })}\n\n`
 }
 
+function chatTools(calls: Array<{ id: string; arguments: string }>): string {
+  return `data: ${JSON.stringify({ choices: [{ index: 0, delta: { tool_calls: calls.map((call, index) => ({
+    index, id: call.id, type: 'function', function: { name: 'FixtureWrite', arguments: call.arguments },
+  })) }, finish_reason: 'tool_calls' }] })}\n\n`
+}
+
 function chatTool(argumentsJson: string, finish?: string): string {
   return `data: ${JSON.stringify({ choices: [{ index: 0, delta: { tool_calls: [{
     index: 0, id: 'call_fixture', type: 'function',
@@ -180,6 +212,33 @@ for (const scenario of scenarios) {
       const resultLine = stdout.split('\n').find(line => line.startsWith(resultPrefix))
       expect(resultLine, stdout + stderr).toBeDefined()
       const result = JSON.parse(resultLine!.slice(resultPrefix.length))
+      if (scenario === 'chat-malformed-corrected' || scenario === 'chat-mixed-corrected' || scenario === 'chat-malformed-repeated') {
+        const mixed = scenario === 'chat-mixed-corrected'
+        const repeated = scenario === 'chat-malformed-repeated'
+        expect(result.requests).toBe(3)
+        expect(result.maxTurnsReached).toBe(repeated)
+        expect(result.executions).toBe(repeated ? 0 : mixed ? 2 : 1)
+        expect(result.committedToolIds).toEqual(repeated
+          ? ['call_invalid_1', 'call_invalid_2', 'call_invalid_3']
+          : mixed ? ['call_valid', 'call_fixture', 'call_corrected'] : ['call_fixture', 'call_corrected'])
+        const invalidId = repeated ? 'call_invalid_1' : 'call_fixture'
+        expect(result.feedback).toContainEqual({ request: 2, id: invalidId, error: true, content: expect.stringContaining('InputValidationError') })
+        const target = join(root, `${scenario}.txt`)
+        if (repeated) {
+          expect(result.executedPaths).toEqual([])
+          expect(await Bun.file(target).exists()).toBe(false)
+          expect(result.feedback).toContainEqual({ request: 3, id: 'call_invalid_2', error: true, content: expect.stringContaining('InputValidationError') })
+        } else {
+          expect(await readFile(target, 'utf8')).toBe('written exactly once')
+          expect(result.executedPaths).toEqual(mixed ? [join(root, 'already-written.txt'), target] : [target])
+          expect(result.feedback).toContainEqual({ request: 3, id: 'call_corrected', error: false, content: JSON.stringify('written') })
+          if (mixed) {
+            expect(await readFile(join(root, 'already-written.txt'), 'utf8')).toBe('written exactly once')
+            expect(result.feedback).toContainEqual({ request: 2, id: 'call_valid', error: false, content: JSON.stringify('written') })
+          }
+        }
+        return
+      }
       const success = scenario === 'chat-completed' || scenario === 'responses-done-only' || scenario === 'anthropic-duplicate'
       expect(result.executions).toBe(success ? 1 : 0)
       expect(result.committedToolIds).toEqual(success ? ['call_fixture'] : [])
diff --git a/src/server/proxy/streaming/openaiChatStreamToAnthropic.test.ts b/src/server/proxy/streaming/openaiChatStreamToAnthropic.test.ts
index 9e5330b3..e5b7828d 100644
--- a/src/server/proxy/streaming/openaiChatStreamToAnthropic.test.ts
+++ b/src/server/proxy/streaming/openaiChatStreamToAnthropic.test.ts
@@ -52,7 +52,7 @@ describe('Chat stream protocol boundaries', () => {
     expect(events.some(e => e.type === 'message_stop')).toBe(false)
   })
 
-  for (const broken of [tool(0, { id: '' }), tool(0, { function: { name: '', arguments: '{}' } }), tool(0, { function: { name: 'Read', arguments: '{"path":' } }), tool(0, { function: { name: 'Read', arguments: '[]' } })]) {
+  for (const broken of [tool(0, { id: '' }), tool(0, { function: { name: '', arguments: '{}' } }), tool(0, { function: { name: 'Read', arguments: '[]' } })]) {
     test(`rejects completed invalid tool ${JSON.stringify(broken)}`, async () => {
       const events = await collect(chunk({ tool_calls: [broken] }) + chunk({}, 'tool_calls'))
       expect(events.at(-1).type).toBe('error')
@@ -60,6 +60,16 @@ describe('Chat stream protocol boundaries', () => {
     })
   }
 
+  test('completed malformed arguments reach CLI validation unchanged', async () => {
+    const raw = '{"path":'
+    const events = await collect(chunk({ tool_calls: [tool(0, { function: { name: 'Read', arguments: raw } })] }) + chunk({}, 'tool_calls'))
+    expect(events.at(-1).type).toBe('message_stop')
+    expect(events.find(e => e.type === 'message_delta').delta.stop_reason).toBe('tool_use')
+    expect(events.find(e => e.content_block?.type === 'tool_use').content_block.id).toBe('call_0')
+    expect(events.filter(e => e.delta?.partial_json).map(e => e.delta.partial_json).join('')).toBe(raw)
+    expect(events.some(e => e.type === 'error')).toBe(false)
+  })
+
   test('length retains truncation cause without synthesizing tool success', async () => {
     const events = await collect(chunk({ tool_calls: [tool(0, { function: { name: 'Read', arguments: '{"path":' } })] }) + chunk({}, 'length'))
     expect(events.find(e => e.type === 'message_delta').delta.stop_reason).toBe('max_tokens')
diff --git a/src/server/proxy/streaming/openaiChatStreamToAnthropic.ts b/src/server/proxy/streaming/openaiChatStreamToAnthropic.ts
index ec0dcfc5..50403751 100644
--- a/src/server/proxy/streaming/openaiChatStreamToAnthropic.ts
+++ b/src/server/proxy/streaming/openaiChatStreamToAnthropic.ts
@@ -517,6 +517,8 @@ function handleFinishReason(
   if (finishReason !== 'length' && finishReason !== 'content_filter') {
     for (const block of state.toolBlocks.values()) {
       if (!block.id.trim() || !block.name.trim()) throw new Error('OpenAI Chat tool call is missing its id or function name')
+      // Syntax errors are tool input errors, not transport failures. Keep the
+      // original deltas so the CLI can return a matching error tool_result.
       parseCompleteChatToolArguments(block.argsBuffer)
     }
     if (stopReason === 'tool_use' && state.toolBlocks.size === 0) {
diff --git a/src/server/proxy/streaming/openaiResponsesContract.test.ts b/src/server/proxy/streaming/openaiResponsesContract.test.ts
index 28003128..c3dec8f4 100644
--- a/src/server/proxy/streaming/openaiResponsesContract.test.ts
+++ b/src/server/proxy/streaming/openaiResponsesContract.test.ts
@@ -82,8 +82,8 @@ describe('Responses tool finalization and snapshots', () => {
     const events = await collect(startTool + event('response.function_call_arguments.delta', { item_id: 'fc_1', delta: '{"path":' }) + completed([tool]))
     expect(events.filter(item => item.delta?.type === 'input_json_delta').map(item => item.delta.partial_json).join('')).toBe(tool.arguments)
   })
-  test('completed malformed, missing and conflicting arguments are rejected', async () => {
-    for (const argumentsValue of ['{"path":', '[]', 'null']) {
+  test('completed non-object, missing and conflicting arguments are rejected', async () => {
+    for (const argumentsValue of ['[]', 'null']) {
       await expect(collect(startTool + event('response.function_call_arguments.delta', { item_id: 'fc_1', delta: argumentsValue }) + completed())).rejects.toThrow('tool arguments')
     }
     await expect(collect(startTool + completed())).rejects.toThrow('tool arguments')
@@ -105,8 +105,8 @@ describe('Responses non-streaming terminal validation', () => {
   test('content_filter is not labeled as a token limit', () => {
     expect(() => openaiResponsesToAnthropic(response({ status: 'incomplete', incomplete_details: { reason: 'content_filter' } }), 'fixture')).toThrow('content_filter')
   })
-  test('malformed completed tool input is not converted into raw or empty objects', () => {
-    for (const argumentsValue of ['{"path":', '', null, '[]']) {
+  test('missing and non-object completed tool input is rejected', () => {
+    for (const argumentsValue of ['', null, '[]']) {
       expect(() => openaiResponsesToAnthropic(response({ output: [{ ...tool, arguments: argumentsValue }] }), 'fixture')).toThrow('tool arguments')
     }
   })
@@ -191,3 +191,24 @@ test('corrupt SSE frames cannot be hidden by a later completed tool snapshot', a
     await expect(openaiResponsesStreamToAnthropicResponse(stream(input), 'fixture', { openAICodexOAuth: oauth })).rejects.toThrow('Invalid OpenAI Responses SSE JSON')
   }
 })
+
+for (const oauth of [false, true]) {
+  test(`completed malformed arguments survive for CLI correction (oauth=${oauth})`, async () => {
+    const raw = '{"path":"fixture","text":"broken\\escape"}'
+    const input = startTool + event('response.function_call_arguments.delta', { item_id: 'fc_1', delta: raw }) + completed([{ ...tool, arguments: raw }])
+    const events = await collect(input, oauth)
+    expect(events.filter(item => item.delta?.type === 'input_json_delta').map(item => item.delta.partial_json).join('')).toBe(raw)
+    expect(events.find(item => item.type === 'message_delta')?.delta.stop_reason).toBe('tool_use')
+    const result = await openaiResponsesStreamToAnthropicResponse(stream(input), 'fixture', { openAICodexOAuth: oauth })
+    expect(result.content.find(item => item.type === 'tool_use')).toMatchObject({ id: 'call_1', input: { __unparsedToolInput: { raw, len: raw.length } } })
+    const buffered = openaiResponsesToAnthropic(response({ output: [{ ...tool, arguments: raw }] }), 'fixture')
+    expect(buffered.content.find(item => item.type === 'tool_use')).toMatchObject({ id: 'call_1', input: { __unparsedToolInput: { raw, len: raw.length } } })
+  })
+}
+
+test('different malformed arguments with identical truncated markers remain a snapshot conflict', async () => {
+  const prefix = '{"text":"' + 'a'.repeat(2200)
+  const delta = prefix + 'X'
+  const final = prefix + 'Y'
+  await expect(collect(startTool + event('response.function_call_arguments.delta', { item_id: 'fc_1', delta }) + completed([{ ...tool, arguments: final }]))).rejects.toThrow('snapshot conflicts')
+})
diff --git a/src/server/proxy/streaming/openaiResponsesStreamToAnthropic.ts b/src/server/proxy/streaming/openaiResponsesStreamToAnthropic.ts
index 5402072d..9fe2ff01 100644
--- a/src/server/proxy/streaming/openaiResponsesStreamToAnthropic.ts
+++ b/src/server/proxy/streaming/openaiResponsesStreamToAnthropic.ts
@@ -586,7 +586,9 @@ function reconcileToolArguments(index: number, value: unknown, state: StreamStat
     // Compatible providers can serialize the same final object differently.
     // Keep the emitted representation when the parsed inputs are identical.
     try {
-      if (isDeepStrictEqual(parseResponsesToolArguments(text), parseResponsesToolArguments(tool.arguments))) return
+      const finalInput = asRecord(JSON.parse(text))
+      const streamedInput = asRecord(JSON.parse(tool.arguments))
+      if (finalInput && streamedInput && isDeepStrictEqual(finalInput, streamedInput)) return
     } catch {
       // Partial JSON cannot establish semantic equality.
     }
diff --git a/src/server/proxy/transform/openaiChatToAnthropic.test.ts b/src/server/proxy/transform/openaiChatToAnthropic.test.ts
index 710fa19f..3b1f9b9a 100644
--- a/src/server/proxy/transform/openaiChatToAnthropic.test.ts
+++ b/src/server/proxy/transform/openaiChatToAnthropic.test.ts
@@ -7,11 +7,20 @@ function response(tool: Record<string, unknown>, reason = 'tool_calls'): OpenAIC
 }
 
 describe('Chat non-streaming response integrity', () => {
-  for (const args of ['{"path":', '[]', 'null', '42']) {
+  for (const args of ['[]', 'null', '42']) {
     test(`completed malformed/non-object arguments reject: ${args}`, () => {
       expect(() => openaiChatToAnthropic(response({ id: 'call', function: { name: 'Read', arguments: args } }), 'fixture')).toThrow()
     })
   }
+  test('completed malformed arguments retain identity and a bounded non-executable marker', () => {
+    const raw = '{"path":"' + 'x'.repeat(3000)
+    const result = openaiChatToAnthropic(response({ id: 'call', function: { name: 'Read', arguments: raw } }), 'fixture')
+    expect(result.stop_reason).toBe('tool_use')
+    expect(result.content[0]).toEqual({
+      type: 'tool_use', id: 'call', name: 'Read',
+      input: { __unparsedToolInput: { raw: raw.slice(0, 2048), len: raw.length } },
+    })
+  })
   test('error envelope rejects rather than returning empty success', () => {
     expect(() => openaiChatToAnthropic({ error: { message: 'Fixture failure' } } as unknown as OpenAIChatResponse, 'fixture')).toThrow('Fixture failure')
   })
diff --git a/src/server/proxy/transform/openaiChatToAnthropic.ts b/src/server/proxy/transform/openaiChatToAnthropic.ts
index e69f51c3..5403fa20 100644
--- a/src/server/proxy/transform/openaiChatToAnthropic.ts
+++ b/src/server/proxy/transform/openaiChatToAnthropic.ts
@@ -10,6 +10,7 @@ import type {
   AnthropicContentBlock,
 } from './types.js'
 import { openaiUsageToAnthropic } from './usage.js'
+import { createUnparsedToolInput } from '../../../utils/unparsedToolInput.js'
 
 /**
  * Convert OpenAI Chat Completions response to Anthropic Messages response.
@@ -99,9 +100,17 @@ function mapFinishReason(reason: string | null): string {
   }
 }
 
-/** Completed tool input must be an object, never a repaired fragment. */
+/** Preserve malformed completed calls for CLI validation, never repair or execute them. */
 export function parseCompleteChatToolArguments(value: unknown): Record<string, unknown> {
-  const parsed: unknown = value == null || value === '' ? {} : typeof value === 'string' ? JSON.parse(value) : value
+  let parsed: unknown = value == null || value === '' ? {} : value
+  if (typeof value === 'string' && value !== '') {
+    try {
+      parsed = JSON.parse(value)
+    } catch (error) {
+      if (!(error instanceof SyntaxError)) throw error
+      return createUnparsedToolInput(value)
+    }
+  }
   if (!parsed || typeof parsed !== 'object' || Array.isArray(parsed)) {
     throw new Error('OpenAI Chat completed tool arguments must be a JSON object')
   }
diff --git a/src/server/proxy/transform/openaiResponsesTerminal.ts b/src/server/proxy/transform/openaiResponsesTerminal.ts
index d850f6c8..c35dbb08 100644
--- a/src/server/proxy/transform/openaiResponsesTerminal.ts
+++ b/src/server/proxy/transform/openaiResponsesTerminal.ts
@@ -1,3 +1,4 @@
+import { createUnparsedToolInput } from '../../../utils/unparsedToolInput.js'
 import { getOpenAIPolicyError } from '../../../services/openaiAuth/policyError.js'
 
 export function responsesRecord(value: unknown): Record<string, unknown> | null {
@@ -39,14 +40,14 @@ export function responsesTerminalStop(
   throw Object.assign(new Error(`OpenAI response has no valid terminal status: ${status ?? 'missing'}`), { type: 'api_error' })
 }
 
-/** Tool input is executable data: malformed JSON must never become {} or raw. */
+/** Preserve complete malformed calls as non-executable input for model correction. */
 export function parseResponsesToolArguments(value: unknown): Record<string, unknown> {
   let parsed = value
   if (typeof value === 'string' && value.trim()) {
     try {
       parsed = JSON.parse(value)
     } catch {
-      throw new Error('Invalid OpenAI Responses tool arguments: incomplete or malformed JSON')
+      return createUnparsedToolInput(value)
     }
   }
   const record = responsesRecord(parsed)
diff --git a/src/services/tools/toolExecution.test.ts b/src/services/tools/toolExecution.test.ts
new file mode 100644
index 00000000..f390e40e
--- /dev/null
+++ b/src/services/tools/toolExecution.test.ts
@@ -0,0 +1,94 @@
+import { expect, mock, test } from 'bun:test'
+import { mkdtemp, rm } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { createSandboxedTestEnvironment } from '../../../scripts/pr/test-environment.js'
+import type { Tool, ToolUseContext } from '../../Tool.js'
+
+const childScenario = process.env.CC_HAHA_TOOL_INPUT_GUARD_SCENARIO
+
+// Importing the executor loads the full tools/permissions/MCP graph. Isolate
+// those caches from shared-process suites that install their own module mocks.
+async function runScenario(root: string, source: string) {
+  ;(globalThis as typeof globalThis & { MACRO?: { BUILD_TIME: string } }).MACRO = { BUILD_TIME: '' }
+  const { z } = await import('zod/v4')
+  const bootstrap = await import('../../bootstrap/state.js')
+  bootstrap.setCwdState(root)
+  bootstrap.setOriginalCwd(root)
+  bootstrap.setProjectRoot(root)
+  process.chdir(root)
+  const { createAssistantMessage, normalizeContentFromAPI } = await import('../../utils/messages.js')
+  const { createUnparsedToolInput } = await import('../../utils/unparsedToolInput.js')
+  const { runToolUse } = await import('./toolExecution.js')
+  const raw = '{"path":' + 'x'.repeat(3000)
+  const marker = createUnparsedToolInput(raw)
+  const normalized = normalizeContentFromAPI([{ type: 'tool_use', id: 'bad-call', name: 'OptionalTool', input: raw }], [])[0]!
+  if (normalized.type !== 'tool_use') throw new Error('Expected tool use')
+  const input = source === 'provider' ? normalized.input : JSON.parse(JSON.stringify(marker))
+  const inputSchema = z.object({ optional: z.string().optional() })
+  expect(inputSchema.safeParse({}).success).toBe(true)
+  const safeParse = mock(inputSchema.safeParse.bind(inputSchema))
+  inputSchema.safeParse = safeParse
+  const call = mock(async () => ({ data: 'must not run' }))
+  const validateInput = mock(async () => ({ result: true }))
+  const tool = { name: 'OptionalTool', inputSchema, call, validateInput } as unknown as Tool
+  const canUseTool = mock(async () => ({ behavior: 'allow' as const, updatedInput: {} }))
+  const toolUse = { type: 'tool_use' as const, id: 'bad-call', name: tool.name, input }
+  const assistant = createAssistantMessage({ content: [toolUse] })
+  const context = { options: { tools: [tool], mcpClients: [] }, abortController: new AbortController(), messages: [] } as unknown as ToolUseContext
+  const updates = []
+  for await (const update of runToolUse(toolUse, assistant, canUseTool, context)) updates.push(update)
+  expect(updates).toHaveLength(1)
+  const message = updates[0]!.message
+  expect(message.type).toBe('user')
+  if (message.type !== 'user') throw new Error('Expected tool error result')
+  expect(message.message.content).toEqual([{
+    type: 'tool_result', tool_use_id: 'bad-call', is_error: true,
+    content: expect.stringContaining('InputValidationError: OptionalTool was called with input that could not be parsed as JSON.'),
+  }])
+  const content = JSON.stringify(message.message.content)
+  expect(content).toContain('Retry with valid JSON')
+  expect(content).toContain(`first 200 of ${raw.length} bytes`)
+  expect(content).not.toContain('x'.repeat(201))
+  expect(message.toolUseResult).toBe(`InputValidationError: JSON parse failed (${raw.length} bytes)`)
+  expect(safeParse).not.toHaveBeenCalled()
+  expect(validateInput).not.toHaveBeenCalled()
+  expect(canUseTool).not.toHaveBeenCalled()
+  expect(call).not.toHaveBeenCalled()
+
+}
+
+for (const scenario of ['provider', 'history']) {
+  if (childScenario && childScenario !== scenario) continue
+  test(`rejects ${scenario} markers before permissive schema, permission checks or tool side effects`, async () => {
+    if (childScenario) {
+      await runScenario(process.env.HOME!, scenario)
+      return
+    }
+    const root = await mkdtemp(join(tmpdir(), 'tool-input-guard-'))
+    const child = Bun.spawn([process.execPath, '--no-env-file', 'test', fileURLToPath(import.meta.url)], {
+      cwd: root,
+      env: createSandboxedTestEnvironment(root, {
+        CC_HAHA_TOOL_INPUT_GUARD_SCENARIO: scenario,
+        NODE_ENV: 'production',
+        CLAUDE_CODE_SIMPLE: '1',
+        CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC: '1',
+        ANTHROPIC_API_KEY: 'offline-fixture-key',
+      }),
+      stdin: 'ignore', stdout: 'pipe', stderr: 'pipe',
+    })
+    const timeout = setTimeout(() => child.kill('SIGKILL'), 15_000)
+    try {
+      const [exitCode, stdout, stderr] = await Promise.all([
+        child.exited, new Response(child.stdout).text(), new Response(child.stderr).text(),
+      ])
+      expect(exitCode, stdout + stderr).toBe(0)
+    } finally {
+      clearTimeout(timeout)
+      child.kill()
+      await child.exited
+      await rm(root, { recursive: true, force: true })
+    }
+  }, 20_000)
+}
diff --git a/src/services/tools/toolExecution.ts b/src/services/tools/toolExecution.ts
index 3c95f98e..02396e79 100644
--- a/src/services/tools/toolExecution.ts
+++ b/src/services/tools/toolExecution.ts
@@ -86,6 +86,7 @@ import {
   stopSessionActivity,
 } from '../../utils/sessionActivity.js'
 import { jsonStringify } from '../../utils/slowOperations.js'
+import { isUnparsedToolInput } from '../../utils/unparsedToolInput.js'
 import { Stream } from '../../utils/stream.js'
 import { logOTelEvent } from '../../utils/telemetry/events.js'
 import {
@@ -341,6 +342,26 @@ export async function* runToolUse(
   toolUseContext: ToolUseContext,
 ): AsyncGenerator<MessageUpdateLazy, void> {
   const toolName = toolUse.name
+  if (isUnparsedToolInput(toolUse.input)) {
+    const { raw, len } = toolUse.input.__unparsedToolInput
+    const preview = raw.slice(0, 200)
+    const errorContent = `${toolName} was called with input that could not be parsed as JSON.\n` +
+      `You sent (first ${preview.length} of ${len} bytes): ${preview}\n` +
+      'Common causes: unescaped backslashes in file paths (use / or \\\\), unescaped control characters, or truncated output. Retry with valid JSON.'
+    yield {
+      message: createUserMessage({
+        content: [{
+          type: 'tool_result',
+          tool_use_id: toolUse.id,
+          is_error: true,
+          content: `<tool_use_error>InputValidationError: ${errorContent}</tool_use_error>`,
+        }],
+        toolUseResult: `InputValidationError: JSON parse failed (${len} bytes)`,
+        sourceToolAssistantUUID: assistantMessage.uuid,
+      }),
+    }
+    return
+  }
   // First try to find in the available tools (what the model sees)
   let tool = findToolByName(toolUseContext.options.tools, toolName)
 
diff --git a/src/utils/messages.test.ts b/src/utils/messages.test.ts
index 4cd3cde0..818a4735 100644
--- a/src/utils/messages.test.ts
+++ b/src/utils/messages.test.ts
@@ -1,10 +1,12 @@
 import { describe, expect, test } from 'bun:test'
 import type { ContentBlockParam } from '@anthropic-ai/sdk/resources/index.mjs'
+import type { Tool } from '../Tool.js'
 import type { AssistantMessage } from '../types/message.js'
 import {
   createAssistantMessage,
   createUserMessage,
   normalizeMessagesForAPI,
+  normalizeContentFromAPI,
   stripSignatureBlocksAfterModelChange,
 } from './messages.js'
 
@@ -229,3 +231,46 @@ describe('stripSignatureBlocksAfterModelChange', () => {
     ).toBe(messages)
   })
 })
+
+describe('malformed provider tool arguments', () => {
+  test('preserves invalid JSON as a bounded marker rather than executable empty input', () => {
+    const raw = '{"path":' + 'x'.repeat(3000)
+    const [block] = normalizeContentFromAPI([{ type: 'tool_use', id: 'bad-call', name: 'TaskList', input: raw }], [])
+    expect(block).toEqual({ type: 'tool_use', id: 'bad-call', name: 'TaskList', input: { __unparsedToolInput: { raw: raw.slice(0, 2048), len: raw.length } } })
+  })
+
+  test('retains existing history markers and still accepts genuine empty input', () => {
+    const marker = { __unparsedToolInput: { raw: '{broken', len: 7 } }
+    const normalized = normalizeContentFromAPI([
+      { type: 'tool_use', id: 'historical', name: 'Read', input: marker },
+      { type: 'tool_use', id: 'empty', name: 'TaskList', input: '' },
+      { type: 'tool_use', id: 'valid', name: 'TaskList', input: '{}' },
+      { type: 'tool_use', id: 'null', name: 'TaskList', input: ' null ' },
+    ], [])
+    expect(normalized.map(block => block.type === 'tool_use' ? block.input : null)).toEqual([marker, {}, {}, {}])
+  })
+})
+
+
+test('malformed and restored markers bypass TaskOutput default injection and survive API history normalization', () => {
+  const tool = { name: 'TaskOutput' } as Tool
+  const raw = '{truncated'
+  const marker = { __unparsedToolInput: { raw, len: raw.length } }
+  for (const input of [raw, marker]) {
+    const blocks = normalizeContentFromAPI([{ type: 'tool_use', id: 'task-output', name: tool.name, input }], [tool])
+    expect(blocks[0]).toMatchObject({ input: marker })
+    const restored = JSON.parse(JSON.stringify(createAssistantMessage({ content: blocks })))
+    const history = normalizeMessagesForAPI([restored, toolResult('task-output')], [tool])
+    expect(history[0]!.message.content).toEqual([{ type: 'tool_use', id: 'task-output', name: tool.name, input: marker }])
+  }
+})
+
+test('legacy ordinary tool-use JSON still round-trips without a migration', () => {
+  const tool = { name: 'Read' } as Tool
+  const historical = JSON.parse(JSON.stringify(assistant('legacy-response', [toolUse('legacy-read')])))
+  const blocks = normalizeContentFromAPI(historical.message.content, [tool])
+  expect(blocks).toEqual(historical.message.content)
+  historical.message.content = blocks
+  const replay = normalizeMessagesForAPI([historical, toolResult('legacy-read')], [tool])
+  expect(replay[0]!.message.content).toEqual(historical.message.content)
+})
diff --git a/src/utils/messages.ts b/src/utils/messages.ts
index 506177b3..812e9449 100644
--- a/src/utils/messages.ts
+++ b/src/utils/messages.ts
@@ -162,6 +162,7 @@ import { TASK_OUTPUT_TOOL_NAME } from '../tools/TaskOutputTool/constants.js'
 import { TASK_UPDATE_TOOL_NAME } from '../tools/TaskUpdateTool/constants.js'
 import type { PermissionMode } from '../types/permissions.js'
 import { normalizeToolInput, normalizeToolInputForAPI } from './api.js'
+import { createUnparsedToolInput, isUnparsedToolInput } from './unparsedToolInput.js'
 import { getCurrentProjectConfig } from './config.js'
 import { logAntError, logForDebugging } from './debug.js'
 import { stripIdeContextTags } from './displayTags.js'
@@ -2273,7 +2274,7 @@ export function normalizeMessagesForAPI(
               content: message.message.content.map(block => {
                 if (block.type === 'tool_use') {
                   const tool = tools.find(t => toolMatchesName(t, block.name))
-                  const normalizedInput = tool
+                  const normalizedInput = tool && !isUnparsedToolInput(block.input)
                     ? normalizeToolInputForAPI(
                         tool,
                         block.input as Record<string, unknown>,
@@ -2852,11 +2853,8 @@ export function normalizeContentFromAPI(
         let normalizedInput: unknown
         if (typeof contentBlock.input === 'string') {
           const parsed = safeParseJSON(contentBlock.input)
-          if (parsed === null && contentBlock.input.length > 0) {
-            // TET/FC-v3 diagnostic: the streamed tool input JSON failed to
-            // parse. We fall back to {} which means downstream validation
-            // sees empty input. The raw prefix goes to debug log only — no
-            // PII-tagged proto column exists for it yet.
+          if (parsed === null && contentBlock.input.trim() !== 'null' && contentBlock.input.length > 0) {
+            normalizedInput = createUnparsedToolInput(contentBlock.input)
             logEvent('tengu_tool_input_json_parse_fail', {
               toolName: sanitizeToolNameForAnalytics(contentBlock.name),
               inputLen: contentBlock.input.length,
@@ -2867,14 +2865,15 @@ export function normalizeContentFromAPI(
                 { level: 'warn' },
               )
             }
+          } else {
+            normalizedInput = parsed ?? {}
           }
-          normalizedInput = parsed ?? {}
         } else {
           normalizedInput = contentBlock.input
         }
 
         // Then apply tool-specific corrections
-        if (typeof normalizedInput === 'object' && normalizedInput !== null) {
+        if (typeof normalizedInput === 'object' && normalizedInput !== null && !isUnparsedToolInput(normalizedInput)) {
           const tool = findToolByName(tools, contentBlock.name)
           if (tool) {
             try {
diff --git a/src/utils/unparsedToolInput.test.ts b/src/utils/unparsedToolInput.test.ts
new file mode 100644
index 00000000..95e7d239
--- /dev/null
+++ b/src/utils/unparsedToolInput.test.ts
@@ -0,0 +1,17 @@
+import { describe, expect, test } from 'bun:test'
+import { createUnparsedToolInput, isUnparsedToolInput } from './unparsedToolInput.js'
+
+describe('unparsed tool input marker', () => {
+  test('bounds the raw argument prefix while retaining original length', () => {
+    const raw = '{' + 'x'.repeat(3000)
+    expect(createUnparsedToolInput(raw)).toEqual({ __unparsedToolInput: { raw: raw.slice(0, 2048), len: raw.length } })
+    expect(isUnparsedToolInput(createUnparsedToolInput(raw))).toBe(true)
+  })
+
+  test.each([null, [], {}, { __unparsedToolInput: null }, { __unparsedToolInput: 'bad' },
+    { __unparsedToolInput: { raw: 'bad', len: '3' } },
+    { __unparsedToolInput: { raw: 'bad', len: 3 }, legitimate: true },
+  ].map(input => [input]))('does not treat ordinary or malformed objects as the reserved marker: %j', input => {
+    expect(isUnparsedToolInput(input)).toBe(false)
+  })
+})
diff --git a/src/utils/unparsedToolInput.ts b/src/utils/unparsedToolInput.ts
new file mode 100644
index 00000000..d27151d2
--- /dev/null
+++ b/src/utils/unparsedToolInput.ts
@@ -0,0 +1,16 @@
+/** Preserve invalid provider arguments without turning them into executable {}. */
+export type UnparsedToolInput = {
+  __unparsedToolInput: { raw: string; len: number }
+}
+
+export function createUnparsedToolInput(raw: string): UnparsedToolInput {
+  return { __unparsedToolInput: { raw: raw.slice(0, 2048), len: raw.length } }
+}
+
+export function isUnparsedToolInput(input: unknown): input is UnparsedToolInput {
+  if (typeof input !== 'object' || input === null || Array.isArray(input)) return false
+  if (Object.keys(input).length !== 1 || !Object.hasOwn(input, '__unparsedToolInput')) return false
+  const value = (input as UnparsedToolInput).__unparsedToolInput
+  return typeof value === 'object' && value !== null
+    && typeof value.raw === 'string' && typeof value.len === 'number'
+}