diff --git a/.ai-sdk-factory/reproduction-replay.json b/.ai-sdk-factory/reproduction-replay.json
new file mode 100644
index 000000000000..6595fb62a087
--- /dev/null
+++ b/.ai-sdk-factory/reproduction-replay.json
@@ -0,0 +1,13 @@
+{
+  "version": 1,
+  "issueId": 18331,
+  "command": "pnpm -C examples/ai-functions exec tsx src/reproduction/issue-18331-anthropic-spliced-message-start.ts",
+  "artifactPaths": [
+    "examples/ai-functions/package.json",
+    "examples/ai-functions/src/reproduction/issue-18331-anthropic-spliced-message-start.ts"
+  ],
+  "expectedFailure": {
+    "exitCode": 1,
+    "signal": "ISSUE_18331_REPRODUCED: different message_start was silently merged into one completed streamText step"
+  }
+}
diff --git a/examples/ai-functions/package.json b/examples/ai-functions/package.json
new file mode 100644
index 000000000000..e7ca7d097254
--- /dev/null
+++ b/examples/ai-functions/package.json
@@ -0,0 +1,5 @@
+{
+  "name": "@example/ai-functions",
+  "version": "0.0.0",
+  "private": true
+}
diff --git a/examples/ai-functions/src/reproduction/issue-18331-anthropic-spliced-message-start.ts b/examples/ai-functions/src/reproduction/issue-18331-anthropic-spliced-message-start.ts
new file mode 100644
index 000000000000..e0d8d59f8eac
--- /dev/null
+++ b/examples/ai-functions/src/reproduction/issue-18331-anthropic-spliced-message-start.ts
@@ -0,0 +1,276 @@
+import { createAnthropic } from '../../../../packages/anthropic/dist/index.mjs';
+import {
+  InvalidResponseDataError,
+  streamText,
+} from '../../../../packages/ai/dist/index.mjs';
+
+type StreamPart = {
+  type: string;
+  id?: string;
+  text?: string;
+  toolCallId?: string;
+  error?: unknown;
+  providerMetadata?: {
+    anthropic?: {
+      signature?: string;
+    };
+  };
+  usage?: {
+    inputTokens?: number;
+  };
+};
+
+const splicedEvents = [
+  {
+    type: 'message_start',
+    message: {
+      id: 'msg_first',
+      model: 'claude-sonnet-4-20250514',
+      usage: { input_tokens: 10, output_tokens: 1 },
+    },
+  },
+  {
+    type: 'content_block_start',
+    index: 0,
+    content_block: { type: 'thinking', thinking: '' },
+  },
+  {
+    type: 'content_block_delta',
+    index: 0,
+    delta: { type: 'thinking_delta', thinking: 'first reasoning' },
+  },
+  {
+    type: 'content_block_delta',
+    index: 0,
+    delta: { type: 'signature_delta', signature: 'sig_first' },
+  },
+  { type: 'content_block_stop', index: 0 },
+  {
+    type: 'content_block_start',
+    index: 1,
+    content_block: {
+      type: 'tool_use',
+      id: 'toolu_first',
+      name: 'lookup',
+      input: {},
+    },
+  },
+  {
+    type: 'content_block_delta',
+    index: 1,
+    delta: { type: 'input_json_delta', partial_json: '{"value":"Spark' },
+  },
+  {
+    type: 'message_start',
+    message: {
+      id: 'msg_second',
+      model: 'claude-sonnet-4-20250514',
+      usage: { input_tokens: 99, output_tokens: 1 },
+    },
+  },
+  {
+    type: 'content_block_start',
+    index: 0,
+    content_block: { type: 'thinking', thinking: '' },
+  },
+  {
+    type: 'content_block_delta',
+    index: 0,
+    delta: { type: 'thinking_delta', thinking: 'second reasoning' },
+  },
+  {
+    type: 'content_block_delta',
+    index: 0,
+    delta: { type: 'signature_delta', signature: 'sig_second' },
+  },
+  { type: 'content_block_stop', index: 0 },
+  {
+    type: 'content_block_start',
+    index: 1,
+    content_block: {
+      type: 'tool_use',
+      id: 'toolu_second',
+      name: 'lookup',
+      input: {},
+    },
+  },
+  {
+    type: 'content_block_delta',
+    index: 1,
+    delta: {
+      type: 'input_json_delta',
+      partial_json: '{"value":"Spark"}',
+    },
+  },
+  { type: 'content_block_stop', index: 1 },
+  {
+    type: 'message_delta',
+    delta: { stop_reason: 'tool_use', stop_sequence: null },
+    usage: { output_tokens: 20 },
+  },
+  { type: 'message_stop' },
+];
+
+const duplicateStartEvents = [
+  {
+    type: 'message_start',
+    message: {
+      id: 'msg_duplicate',
+      model: 'claude-sonnet-4-20250514',
+      usage: { input_tokens: 10, output_tokens: 1 },
+    },
+  },
+  {
+    type: 'message_start',
+    message: {
+      id: 'msg_duplicate',
+      model: 'claude-sonnet-4-20250514',
+      usage: { input_tokens: 99, output_tokens: 1 },
+    },
+  },
+  {
+    type: 'content_block_start',
+    index: 0,
+    content_block: { type: 'text', text: '' },
+  },
+  {
+    type: 'content_block_delta',
+    index: 0,
+    delta: { type: 'text_delta', text: 'ok' },
+  },
+  { type: 'content_block_stop', index: 0 },
+  {
+    type: 'message_delta',
+    delta: { stop_reason: 'end_turn', stop_sequence: null },
+    usage: { output_tokens: 2 },
+  },
+  { type: 'message_stop' },
+];
+
+function createSseFetch(events: unknown[]) {
+  return async () =>
+    new Response(
+      events.map(event => `data: ${JSON.stringify(event)}\n\n`).join(''),
+      {
+        headers: { 'content-type': 'text/event-stream' },
+        status: 200,
+      },
+    );
+}
+
+async function readProviderParts(events: unknown[]): Promise<StreamPart[]> {
+  const model = createAnthropic({
+    apiKey: 'test-api-key',
+    fetch: createSseFetch(events),
+  })('claude-sonnet-4-20250514');
+  const { stream } = await model.doStream({
+    prompt: [{ role: 'user', content: [{ type: 'text', text: 'Hello' }] }],
+  });
+  const parts: StreamPart[] = [];
+  for await (const part of stream) {
+    parts.push(part as StreamPart);
+  }
+  return parts;
+}
+
+async function main() {
+  const providerParts = await readProviderParts(splicedEvents);
+  const providerErrors = providerParts.filter(part => part.type === 'error');
+  const metadataIds = providerParts
+    .filter(part => part.type === 'response-metadata')
+    .map(part => part.id);
+  const completedToolIds = providerParts
+    .filter(part => part.type === 'tool-call')
+    .map(part => part.toolCallId);
+
+  const model = createAnthropic({
+    apiKey: 'test-api-key',
+    fetch: createSseFetch(splicedEvents),
+  })('claude-sonnet-4-20250514');
+  const result = streamText({
+    model,
+    prompt: 'Hello',
+    maxRetries: 0,
+  });
+  const fullStreamParts: StreamPart[] = [];
+  for await (const part of result.fullStream) {
+    fullStreamParts.push(part as StreamPart);
+  }
+  const steps = await result.steps;
+
+  const duplicateParts = await readProviderParts(duplicateStartEvents);
+  const duplicateMetadataCount = duplicateParts.filter(
+    part => part.type === 'response-metadata',
+  ).length;
+  const duplicateInputTokens = duplicateParts.find(
+    part => part.type === 'finish',
+  )?.usage?.inputTokens;
+
+  const invalidResponseError = providerErrors.some(part =>
+    InvalidResponseDataError.isInstance(part.error),
+  );
+  const mergedReasoning = fullStreamParts
+    .filter(part => part.type === 'reasoning-delta')
+    .map(part => part.text)
+    .join('|');
+  const mergedSignatures = fullStreamParts
+    .filter(part => part.type === 'reasoning-delta')
+    .map(part => part.providerMetadata?.anthropic?.signature)
+    .filter(signature => signature != null);
+
+  console.log(
+    JSON.stringify(
+      {
+        metadataIds,
+        invalidResponseError,
+        completedToolIds,
+        mergedReasoning,
+        mergedSignatures,
+        recordedStepCount: steps.length,
+        duplicateMetadataCount,
+        duplicateInputTokens,
+      },
+      null,
+      2,
+    ),
+  );
+
+  if (
+    !invalidResponseError &&
+    metadataIds.join(',') === 'msg_first,msg_second' &&
+    completedToolIds.join(',') === 'toolu_second' &&
+    mergedReasoning === 'first reasoning||second reasoning|' &&
+    mergedSignatures.join(',') === 'sig_first,sig_second' &&
+    steps.length === 1
+  ) {
+    console.error(
+      'ISSUE_18331_REPRODUCED: different message_start was silently merged into one completed streamText step',
+    );
+    process.exitCode = 1;
+    return;
+  }
+
+  if (
+    invalidResponseError &&
+    duplicateMetadataCount === 1 &&
+    duplicateInputTokens === 10
+  ) {
+    console.log(
+      'Issue behavior is fixed: the splice failed explicitly and the duplicate start was ignored.',
+    );
+    return;
+  }
+
+  throw new Error(
+    `Unexpected result: invalidResponseError=${invalidResponseError}, metadataIds=${metadataIds.join(
+      ',',
+    )}, completedToolIds=${completedToolIds.join(
+      ',',
+    )}, recordedStepCount=${steps.length}, duplicateMetadataCount=${duplicateMetadataCount}, duplicateInputTokens=${duplicateInputTokens}`,
+  );
+}
+
+main().catch(error => {
+  console.error(error);
+  process.exitCode = 1;
+});
diff --git a/packages/anthropic/src/__fixtures__/issue-18331-duplicate-message-start.chunks.txt b/packages/anthropic/src/__fixtures__/issue-18331-duplicate-message-start.chunks.txt
new file mode 100644
index 000000000000..defbaad0cd0a
--- /dev/null
+++ b/packages/anthropic/src/__fixtures__/issue-18331-duplicate-message-start.chunks.txt
@@ -0,0 +1,7 @@
+{"type":"message_start","message":{"id":"msg_duplicate","model":"claude-sonnet-4-20250514","usage":{"input_tokens":10,"output_tokens":1}}}
+{"type":"message_start","message":{"id":"msg_duplicate","model":"claude-sonnet-4-20250514","usage":{"input_tokens":99,"output_tokens":1}}}
+{"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}
+{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"ok"}}
+{"type":"content_block_stop","index":0}
+{"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"output_tokens":2}}
+{"type":"message_stop"}
diff --git a/packages/anthropic/src/__fixtures__/issue-18331-sequential-messages.chunks.txt b/packages/anthropic/src/__fixtures__/issue-18331-sequential-messages.chunks.txt
new file mode 100644
index 000000000000..c724c5e7ae2d
--- /dev/null
+++ b/packages/anthropic/src/__fixtures__/issue-18331-sequential-messages.chunks.txt
@@ -0,0 +1,12 @@
+{"type":"message_start","message":{"id":"msg_first","model":"claude-sonnet-4-20250514","usage":{"input_tokens":10,"output_tokens":1}}}
+{"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}
+{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"first"}}
+{"type":"content_block_stop","index":0}
+{"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"output_tokens":2}}
+{"type":"message_stop"}
+{"type":"message_start","message":{"id":"msg_second","model":"claude-sonnet-4-20250514","usage":{"input_tokens":11,"output_tokens":1}}}
+{"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}
+{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"second"}}
+{"type":"content_block_stop","index":0}
+{"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"output_tokens":3}}
+{"type":"message_stop"}
diff --git a/packages/anthropic/src/__fixtures__/issue-18331-spliced-message-start.chunks.txt b/packages/anthropic/src/__fixtures__/issue-18331-spliced-message-start.chunks.txt
new file mode 100644
index 000000000000..2c31681d05ab
--- /dev/null
+++ b/packages/anthropic/src/__fixtures__/issue-18331-spliced-message-start.chunks.txt
@@ -0,0 +1,17 @@
+{"type":"message_start","message":{"id":"msg_first","model":"claude-sonnet-4-20250514","usage":{"input_tokens":10,"output_tokens":1}}}
+{"type":"content_block_start","index":0,"content_block":{"type":"thinking","thinking":""}}
+{"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":"first reasoning"}}
+{"type":"content_block_delta","index":0,"delta":{"type":"signature_delta","signature":"sig_first"}}
+{"type":"content_block_stop","index":0}
+{"type":"content_block_start","index":1,"content_block":{"type":"tool_use","id":"toolu_first","name":"lookup","input":{}}}
+{"type":"content_block_delta","index":1,"delta":{"type":"input_json_delta","partial_json":"{\"value\":\"Spark"}}
+{"type":"message_start","message":{"id":"msg_second","model":"claude-sonnet-4-20250514","usage":{"input_tokens":99,"output_tokens":1}}}
+{"type":"content_block_start","index":0,"content_block":{"type":"thinking","thinking":""}}
+{"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":"second reasoning"}}
+{"type":"content_block_delta","index":0,"delta":{"type":"signature_delta","signature":"sig_second"}}
+{"type":"content_block_stop","index":0}
+{"type":"content_block_start","index":1,"content_block":{"type":"tool_use","id":"toolu_second","name":"lookup","input":{}}}
+{"type":"content_block_delta","index":1,"delta":{"type":"input_json_delta","partial_json":"{\"value\":\"Spark\"}"}}
+{"type":"content_block_stop","index":1}
+{"type":"message_delta","delta":{"stop_reason":"tool_use","stop_sequence":null},"usage":{"output_tokens":20}}
+{"type":"message_stop"}
diff --git a/packages/anthropic/src/issue-18331.test.ts b/packages/anthropic/src/issue-18331.test.ts
new file mode 100644
index 000000000000..dcfbf37741d1
--- /dev/null
+++ b/packages/anthropic/src/issue-18331.test.ts
@@ -0,0 +1,84 @@
+import { InvalidResponseDataError } from '@ai-sdk/provider';
+import { createTestServer } from '@ai-sdk/test-server/with-vitest';
+import { convertReadableStreamToArray } from '@ai-sdk/provider-utils/test';
+import fs from 'node:fs';
+import { describe, expect, it } from 'vitest';
+import { createAnthropic } from './anthropic-provider';
+
+const prompt = [
+  {
+    role: 'user' as const,
+    content: [{ type: 'text' as const, text: 'Hello' }],
+  },
+];
+
+describe('issue 18331', () => {
+  const server = createTestServer({
+    'https://api.anthropic.com/v1/messages': {},
+  });
+  const model = createAnthropic({ apiKey: 'test-api-key' })(
+    'claude-sonnet-4-20250514',
+  );
+
+  function prepareFixture(filename: string) {
+    const chunks = fs
+      .readFileSync(`src/__fixtures__/${filename}.chunks.txt`, 'utf8')
+      .trimEnd()
+      .split('\n')
+      .map(line => `data: ${line}\n\n`);
+
+    server.urls['https://api.anthropic.com/v1/messages'].response = {
+      type: 'stream-chunks',
+      chunks,
+    };
+  }
+
+  it('fails a different message_start while the previous message is open', async () => {
+    prepareFixture('issue-18331-spliced-message-start');
+
+    const { stream } = await model.doStream({ prompt });
+    const parts = await convertReadableStreamToArray(stream);
+
+    expect(
+      parts.some(
+        part =>
+          part.type === 'error' &&
+          InvalidResponseDataError.isInstance(part.error),
+      ),
+    ).toBe(true);
+    expect(parts.filter(part => part.type === 'response-metadata')).toEqual([
+      expect.objectContaining({ id: 'msg_first' }),
+    ]);
+    expect(parts.some(part => part.type === 'finish')).toBe(false);
+    expect(parts.some(part => part.type === 'tool-call')).toBe(false);
+  });
+
+  it('ignores a duplicate message_start for the same open message', async () => {
+    prepareFixture('issue-18331-duplicate-message-start');
+
+    const { stream } = await model.doStream({ prompt });
+    const parts = await convertReadableStreamToArray(stream);
+
+    expect(parts.filter(part => part.type === 'response-metadata')).toEqual([
+      expect.objectContaining({ id: 'msg_duplicate' }),
+    ]);
+    expect(parts.find(part => part.type === 'finish')).toMatchObject({
+      usage: { inputTokens: 10, outputTokens: 2, totalTokens: 12 },
+    });
+  });
+
+  it('keeps sequential messages separated by message_stop valid', async () => {
+    prepareFixture('issue-18331-sequential-messages');
+
+    const { stream } = await model.doStream({ prompt });
+    const parts = await convertReadableStreamToArray(stream);
+
+    expect(
+      parts
+        .filter(part => part.type === 'response-metadata')
+        .map(part => part.id),
+    ).toEqual(['msg_first', 'msg_second']);
+    expect(parts.filter(part => part.type === 'finish')).toHaveLength(2);
+    expect(parts.filter(part => part.type === 'error')).toHaveLength(0);
+  });
+});