diff --git a/packages/agent-runtime/src/__tests__/tool-validation-error.test.ts b/packages/agent-runtime/src/__tests__/tool-validation-error.test.ts
index d3d1d65bd..eb982d368 100644
--- a/packages/agent-runtime/src/__tests__/tool-validation-error.test.ts
+++ b/packages/agent-runtime/src/__tests__/tool-validation-error.test.ts
@@ -233,6 +233,152 @@ describe('tool validation error handling', () => {
     expect(errorEvents.length).toBe(0)
   })
 
+  it('should parse input JSON string from AI SDK before validation', async () => {
+    // The AI SDK can emit tool-call chunks with `input` as a raw JSON string
+    // when upstream schema validation fails and the repair function returns
+    // the original tool call unchanged. The stream parser should parse the
+    // string into an object before handing it to the tool executor.
+    const agentWithReadFiles: AgentTemplate = {
+      ...testAgentTemplate,
+      toolNames: ['read_files', 'end_turn'],
+    }
+
+    const stringInputToolCallChunk = {
+      type: 'tool-call' as const,
+      toolName: 'read_files',
+      toolCallId: 'string-input-tool-call-id',
+      input: JSON.stringify({ paths: ['test.ts'] }) as any,
+    }
+
+    async function* mockStream() {
+      yield stringInputToolCallChunk
+      return promptSuccess('mock-message-id')
+    }
+
+    const sessionState = getInitialSessionState(mockFileContext)
+    const agentState = sessionState.mainAgentState
+
+    agentRuntimeImpl.requestFiles = async () => ({
+      'test.ts': 'console.log("test")',
+    })
+
+    const responseChunks: (string | PrintModeEvent)[] = []
+
+    await processStream({
+      ...agentRuntimeImpl,
+      agentContext: {},
+      agentState,
+      agentStepId: 'test-step-id',
+      agentTemplate: agentWithReadFiles,
+      ancestorRunIds: [],
+      clientSessionId: 'test-session',
+      fileContext: mockFileContext,
+      fingerprintId: 'test-fingerprint',
+      fullResponse: '',
+      localAgentTemplates: { 'test-agent': agentWithReadFiles },
+      messages: [],
+      prompt: 'test prompt',
+      repoId: undefined,
+      repoUrl: undefined,
+      runId: 'test-run-id',
+      signal: new AbortController().signal,
+      stream: mockStream(),
+      system: 'test system',
+      tools: {},
+      userId: 'test-user',
+      userInputId: 'test-input-id',
+      onCostCalculated: async () => {},
+      onResponseChunk: (chunk) => {
+        responseChunks.push(chunk)
+      },
+    })
+
+    const toolCallEvents = responseChunks.filter(
+      (chunk): chunk is Extract<PrintModeEvent, { type: 'tool_call' }> =>
+        typeof chunk !== 'string' && chunk.type === 'tool_call',
+    )
+    expect(toolCallEvents.length).toBe(1)
+    expect(toolCallEvents[0].toolName).toBe('read_files')
+    expect(toolCallEvents[0].input).toEqual({ paths: ['test.ts'] })
+
+    const errorEvents = responseChunks.filter(
+      (chunk): chunk is Extract<PrintModeEvent, { type: 'error' }> =>
+        typeof chunk !== 'string' && chunk.type === 'error',
+    )
+    expect(errorEvents.length).toBe(0)
+  })
+
+  it('should emit a clear error when tool input is an unparseable string', async () => {
+    const agentWithReadFiles: AgentTemplate = {
+      ...testAgentTemplate,
+      toolNames: ['read_files', 'end_turn'],
+    }
+
+    const invalidStringToolCallChunk = {
+      type: 'tool-call' as const,
+      toolName: 'read_files',
+      toolCallId: 'invalid-string-tool-call-id',
+      input: '{"paths": ["test.ts"' as any, // truncated/malformed JSON
+    }
+
+    async function* mockStream() {
+      yield invalidStringToolCallChunk
+      return promptSuccess('mock-message-id')
+    }
+
+    const sessionState = getInitialSessionState(mockFileContext)
+    const agentState = sessionState.mainAgentState
+
+    const responseChunks: (string | PrintModeEvent)[] = []
+
+    const result = await processStream({
+      ...agentRuntimeImpl,
+      agentContext: {},
+      agentState,
+      agentStepId: 'test-step-id',
+      agentTemplate: agentWithReadFiles,
+      ancestorRunIds: [],
+      clientSessionId: 'test-session',
+      fileContext: mockFileContext,
+      fingerprintId: 'test-fingerprint',
+      fullResponse: '',
+      localAgentTemplates: { 'test-agent': agentWithReadFiles },
+      messages: [],
+      prompt: 'test prompt',
+      repoId: undefined,
+      repoUrl: undefined,
+      runId: 'test-run-id',
+      signal: new AbortController().signal,
+      stream: mockStream(),
+      system: 'test system',
+      tools: {},
+      userId: 'test-user',
+      userInputId: 'test-input-id',
+      onCostCalculated: async () => {},
+      onResponseChunk: (chunk) => {
+        responseChunks.push(chunk)
+      },
+    })
+
+    const errorEvents = responseChunks.filter(
+      (chunk): chunk is Extract<PrintModeEvent, { type: 'error' }> =>
+        typeof chunk !== 'string' && chunk.type === 'error',
+    )
+    expect(errorEvents.length).toBe(1)
+    expect(errorEvents[0].message).toContain(
+      'tool arguments were a string, not a JSON object',
+    )
+    expect(errorEvents[0].message).toContain('Original tool call input:')
+
+    expect(result.hadToolCallError).toBe(true)
+
+    const toolCallEvents = responseChunks.filter(
+      (chunk): chunk is Extract<PrintModeEvent, { type: 'tool_call' }> =>
+        typeof chunk !== 'string' && chunk.type === 'tool_call',
+    )
+    expect(toolCallEvents.length).toBe(0)
+  })
+
   it('should preserve tool_call/tool_result ordering when custom tool setup is async', async () => {
     const toolName = 'delayed_custom_tool'
     const agentWithCustomTool: AgentTemplate = {
diff --git a/packages/agent-runtime/src/tool-stream-parser.ts b/packages/agent-runtime/src/tool-stream-parser.ts
index 82a37111b..cd4ca58df 100644
--- a/packages/agent-runtime/src/tool-stream-parser.ts
+++ b/packages/agent-runtime/src/tool-stream-parser.ts
@@ -77,7 +77,17 @@ export async function* processStreamWithTools(params: {
     input: any
     contents?: string
   }): Promise<void> {
-    const { toolName, input, contents } = params
+    const { toolName, contents } = params
+    let { input } = params
+
+    // AI SDK sometimes emits tool-call chunks with a raw JSON string as `input`
+    // when its repair pass can't produce a parsed object. Try to parse; if it
+    // fails, leave as string — the executor surfaces a clear error.
+    if (typeof input === 'string') {
+      try {
+        input = JSON.parse(input)
+      } catch {}
+    }
 
     const processor = processors[toolName] ?? defaultProcessor(toolName)
 
diff --git a/packages/agent-runtime/src/tools/tool-executor.ts b/packages/agent-runtime/src/tools/tool-executor.ts
index da0cfbd3b..78906f4ab 100644
--- a/packages/agent-runtime/src/tools/tool-executor.ts
+++ b/packages/agent-runtime/src/tools/tool-executor.ts
@@ -51,6 +51,18 @@ export type ToolCallError = {
   error: string
 } & Pick<CodebuffToolCall, 'toolCallId'>
 
+function stringInputError(
+  toolName: string,
+  toolCallId: string,
+): ToolCallError {
+  return {
+    toolName,
+    toolCallId,
+    input: {},
+    error: `Invalid parameters for ${toolName}: tool arguments were a string, not a JSON object. This usually means the model emitted malformed JSON (e.g. unescaped newlines or quotes inside a string value). Re-issue the tool call with properly escaped JSON.`,
+  }
+}
+
 export function parseRawToolCall<T extends ToolName = ToolName>(params: {
   rawToolCall: {
     toolName: T
@@ -64,6 +76,10 @@ export function parseRawToolCall<T extends ToolName = ToolName>(params: {
   const processedParameters = rawToolCall.input
   const paramsSchema = toolParams[toolName].inputSchema
 
+  if (typeof processedParameters === 'string') {
+    return stringInputError(toolName, rawToolCall.toolCallId)
+  }
+
   const result = paramsSchema.safeParse(processedParameters)
 
   if (!result.success) {
@@ -388,6 +404,10 @@ export function parseRawCustomToolCall(params: {
     }
   }
 
+  if (typeof rawToolCall.input === 'string') {
+    return stringInputError(toolName, rawToolCall.toolCallId)
+  }
+
   const processedParameters: Record<string, any> = {}
   for (const [param, val] of Object.entries(rawToolCall.input ?? {})) {
     processedParameters[param] = val