diff --git a/.changeset/tidy-claude-subagents.md b/.changeset/tidy-claude-subagents.md
new file mode 100644
index 000000000000..586a8949595b
--- /dev/null
+++ b/.changeset/tidy-claude-subagents.md
@@ -0,0 +1,5 @@
+---
+'@ai-sdk/harness-claude-code': patch
+---
+
+Exclude Claude Code subagent messages from parent step usage and tool-call tracking.
diff --git a/packages/harness-claude-code/src/bridge/__fixtures__/subagent-step-stream.json b/packages/harness-claude-code/src/bridge/__fixtures__/subagent-step-stream.json
new file mode 100644
index 000000000000..593814c5f6e7
--- /dev/null
+++ b/packages/harness-claude-code/src/bridge/__fixtures__/subagent-step-stream.json
@@ -0,0 +1,129 @@
+[
+  {
+    "type": "assistant",
+    "parent_tool_use_id": null,
+    "message": {
+      "content": [{ "type": "thinking", "thinking": "Delegating." }],
+      "usage": {
+        "input_tokens": 2,
+        "cache_creation_input_tokens": 5605,
+        "cache_read_input_tokens": 2298,
+        "output_tokens": 5
+      }
+    }
+  },
+  {
+    "type": "assistant",
+    "parent_tool_use_id": null,
+    "message": {
+      "content": [
+        {
+          "type": "tool_use",
+          "id": "toolu_main_agent",
+          "name": "Agent",
+          "input": {
+            "description": "Run pwd and echo",
+            "prompt": "Use Bash to run pwd, then echo subagent-done.",
+            "subagent_type": "general-purpose",
+            "run_in_background": false
+          }
+        }
+      ],
+      "usage": {
+        "input_tokens": 2,
+        "cache_creation_input_tokens": 5605,
+        "cache_read_input_tokens": 2298,
+        "output_tokens": 5
+      }
+    }
+  },
+  {
+    "type": "user",
+    "parent_tool_use_id": "toolu_main_agent",
+    "message": {
+      "content": [{ "type": "text", "text": "The subagent started." }]
+    }
+  },
+  {
+    "type": "assistant",
+    "parent_tool_use_id": "toolu_main_agent",
+    "message": {
+      "content": [
+        {
+          "type": "tool_use",
+          "id": "toolu_subagent_pwd",
+          "name": "Bash",
+          "input": { "command": "pwd" }
+        }
+      ],
+      "usage": {
+        "input_tokens": 2,
+        "cache_creation_input_tokens": 5024,
+        "cache_read_input_tokens": 3519,
+        "output_tokens": 4
+      }
+    }
+  },
+  {
+    "type": "assistant",
+    "parent_tool_use_id": "toolu_main_agent",
+    "message": {
+      "content": [
+        {
+          "type": "tool_use",
+          "id": "toolu_subagent_echo",
+          "name": "Bash",
+          "input": { "command": "echo subagent-done" }
+        }
+      ],
+      "usage": {
+        "input_tokens": 2,
+        "cache_creation_input_tokens": 5024,
+        "cache_read_input_tokens": 3519,
+        "output_tokens": 4
+      }
+    }
+  },
+  {
+    "type": "user",
+    "parent_tool_use_id": "toolu_main_agent",
+    "message": {
+      "content": [
+        {
+          "type": "tool_result",
+          "tool_use_id": "toolu_subagent_pwd",
+          "content": "/work/packages/harness-claude-code",
+          "is_error": false
+        }
+      ]
+    }
+  },
+  {
+    "type": "user",
+    "parent_tool_use_id": "toolu_main_agent",
+    "message": {
+      "content": [
+        {
+          "type": "tool_result",
+          "tool_use_id": "toolu_subagent_echo",
+          "content": "subagent-done",
+          "is_error": false
+        }
+      ]
+    }
+  },
+  {
+    "type": "user",
+    "parent_tool_use_id": null,
+    "message": {
+      "content": [
+        {
+          "type": "tool_result",
+          "tool_use_id": "toolu_main_agent",
+          "content": "Both commands ran successfully.",
+          "is_error": false
+        }
+      ]
+    }
+  }
+]
diff --git a/packages/harness-claude-code/src/bridge/create-emit-stream-event.test.ts b/packages/harness-claude-code/src/bridge/create-emit-stream-event.test.ts
index 4ca78fc7eaa6..036ae5823cc3 100644
--- a/packages/harness-claude-code/src/bridge/create-emit-stream-event.test.ts
+++ b/packages/harness-claude-code/src/bridge/create-emit-stream-event.test.ts
@@ -1,5 +1,7 @@
+import { readFileSync } from 'node:fs';
 import { describe, expect, it } from 'vitest';
 import {
+  type ClaudeMessage,
   createClaudeStreamEventState,
   createEmitStreamEvent,
 } from './create-emit-stream-event';
@@ -92,6 +94,51 @@ describe('createEmitStreamEvent', () => {
     `);
   });
 
+  it('keeps subagent messages out of the main Agent-tool step', () => {
+    const messages = JSON.parse(
+      readFileSync(
+        new URL('./__fixtures__/subagent-step-stream.json', import.meta.url),
+        'utf8',
+      ),
+    ) as ClaudeMessage[];
+    const state = createClaudeStreamEventState();
+    const emitted: Record<string, unknown>[] = [];
+    const emitStreamEvent = createEmitStreamEvent({
+      state,
+      emit: event => emitted.push(event),
+      emitWarning: () => {},
+      emitTerminalError: () => {},
+      onCompactionBoundary: () => {},
+      toCommonName: name => (name === 'Bash' ? 'bash' : name),
+    });
+
+    for (const message of messages) {
+      emitStreamEvent(message);
+    }
+
+    const finishStep = emitted.find(event => event.type === 'finish-step');
+    const leakedSubagentEvents = emitted.filter(
+      event =>
+        (event.type === 'tool-call' || event.type === 'tool-result') &&
+        typeof event.toolCallId === 'string' &&
+        event.toolCallId.startsWith('toolu_subagent_'),
+    );
+
+    expect(finishStep?.usage).toEqual({
+      inputTokens: {
+        total: 7905,
+        noCache: 2,
+        cacheRead: 2298,
+        cacheWrite: 5605,
+      },
+      outputTokens: {
+        total: 5,
+        text: 5,
+      },
+    });
+    expect(leakedSubagentEvents).toEqual([]);
+  });
+
   it('preserves retry and compaction handling', () => {
     const state = createClaudeStreamEventState();
     const warnings: unknown[] = [];
diff --git a/packages/harness-claude-code/src/bridge/create-emit-stream-event.ts b/packages/harness-claude-code/src/bridge/create-emit-stream-event.ts
index c17e6759aaa8..681cdea139a3 100644
--- a/packages/harness-claude-code/src/bridge/create-emit-stream-event.ts
+++ b/packages/harness-claude-code/src/bridge/create-emit-stream-event.ts
@@ -5,6 +5,7 @@ type Emit = (message: Record<string, unknown>) => void;
 export type ClaudeMessage = {
   type?: string;
   subtype?: string;
+  parent_tool_use_id?: string | null;
   model?: string;
   error?: string;
   error_status?: number | null;
@@ -186,6 +187,10 @@ export function createEmitStreamEvent({
       return;
     }
 
+    if (type === 'assistant' && msg.parent_tool_use_id != null) {
+      return;
+    }
+
     if (type === 'assistant' && msg.message?.content) {
       const usage = mapUsage(msg.message.usage);
       const toolUseIds: string[] = [];
@@ -227,6 +232,10 @@ export function createEmitStreamEvent({
       return;
     }
 
+    if (type === 'user' && msg.parent_tool_use_id != null) {
+      return;
+    }
+
     if (type === 'user' && msg.message?.content) {
       for (const block of msg.message.content) {
         if (