diff --git a/.ai-sdk-factory/reproduction-replay.json b/.ai-sdk-factory/reproduction-replay.json
new file mode 100644
index 000000000000..5eafed8943b3
--- /dev/null
+++ b/.ai-sdk-factory/reproduction-replay.json
@@ -0,0 +1,14 @@
+{
+  "version": 1,
+  "issueId": 18872,
+  "command": "pnpm -C examples/ai-functions exec tsx src/reproduction/open-responses-truncated-reasoning.ts",
+  "artifactPaths": [
+    "examples/ai-functions/src/reproduction/open-responses-truncated-reasoning.ts",
+    "packages/open-responses/src/responses/__fixtures__/issue-18872-truncated-reasoning.chunks.txt",
+    "packages/open-responses/src/responses/__fixtures__/issue-18872-unknown-incomplete-reason.chunks.txt"
+  ],
+  "expectedFailure": {
+    "exitCode": 1,
+    "signal": "ISSUE #18872: Open Responses truncated-stream handling is incorrect"
+  }
+}
diff --git a/examples/ai-functions/src/reproduction/open-responses-truncated-reasoning.ts b/examples/ai-functions/src/reproduction/open-responses-truncated-reasoning.ts
new file mode 100644
index 000000000000..28b8291d67c6
--- /dev/null
+++ b/examples/ai-functions/src/reproduction/open-responses-truncated-reasoning.ts
@@ -0,0 +1,119 @@
+import { createOpenResponses } from '@ai-sdk/open-responses';
+import { streamText, tool } from 'ai';
+import fs from 'node:fs';
+import { z } from 'zod/v4';
+
+function createSseFetch(fixtureName: string): typeof fetch {
+  return async () =>
+    new Response(
+      [
+        ...fs
+          .readFileSync(
+            `../../packages/open-responses/src/responses/__fixtures__/${fixtureName}.chunks.txt`,
+            'utf8',
+          )
+          .trim()
+          .split('\n')
+          .map(event => `data: ${event}\n\n`),
+        'data: [DONE]\n\n',
+      ].join(''),
+      {
+        headers: {
+          'content-type': 'text/event-stream',
+        },
+      },
+    );
+}
+
+async function reproduceMismatchedReasoningEnd() {
+  const openResponses = createOpenResponses({
+    name: 'issue-18872-reasoning',
+    url: 'https://example.test/v1/responses',
+    fetch: createSseFetch('issue-18872-truncated-reasoning'),
+  });
+
+  const result = streamText({
+    model: openResponses('reasoning-model'),
+    prompt: 'Think until the output token limit is reached.',
+  });
+
+  const errors: unknown[] = [];
+
+  for await (const part of result.fullStream) {
+    if (part.type === 'error') {
+      errors.push(part.error);
+    }
+  }
+
+  return {
+    errors,
+    finishReason: await result.finishReason,
+  };
+}
+
+async function reproduceUnknownIncompleteReason() {
+  const openResponses = createOpenResponses({
+    name: 'issue-18872-finish-reason',
+    url: 'https://example.test/v1/responses',
+    fetch: createSseFetch('issue-18872-unknown-incomplete-reason'),
+  });
+
+  const result = streamText({
+    model: openResponses('tool-model'),
+    prompt: 'Check the weather.',
+    tools: {
+      get_weather: tool({
+        inputSchema: z.object({ location: z.string() }),
+        execute: async ({ location }) => ({ location, temperature: 20 }),
+      }),
+    },
+  });
+
+  for await (const _part of result.fullStream) {
+    // Consume the stream so the terminal finish reason is available.
+  }
+
+  return {
+    finishReason: await result.finishReason,
+    rawFinishReason: await result.rawFinishReason,
+  };
+}
+
+async function main() {
+  const reasoning = await reproduceMismatchedReasoningEnd();
+  const unknownReason = await reproduceUnknownIncompleteReason();
+  const failures: string[] = [];
+
+  if (reasoning.errors.length > 0) {
+    failures.push(
+      `truncated reasoning emitted stream errors: ${reasoning.errors.map(String).join(', ')}`,
+    );
+  }
+
+  if (reasoning.finishReason !== 'length') {
+    failures.push(
+      `max_output_tokens mapped to ${JSON.stringify(reasoning.finishReason)} instead of "length"`,
+    );
+  }
+
+  if (unknownReason.finishReason !== 'other') {
+    failures.push(
+      `unknown incomplete reason ${JSON.stringify(unknownReason.rawFinishReason)} mapped to ${JSON.stringify(unknownReason.finishReason)} instead of "other"`,
+    );
+  }
+
+  if (failures.length > 0) {
+    console.error(
+      `ISSUE #18872: Open Responses truncated-stream handling is incorrect\n${failures.map(failure => `- ${failure}`).join('\n')}`,
+    );
+    process.exitCode = 1;
+    return;
+  }
+
+  console.log('Issue #18872 is not reproduced.');
+}
+
+main().catch(error => {
+  console.error(error);
+  process.exitCode = 1;
+});
diff --git a/packages/open-responses/src/responses/__fixtures__/issue-18872-truncated-reasoning.chunks.txt b/packages/open-responses/src/responses/__fixtures__/issue-18872-truncated-reasoning.chunks.txt
new file mode 100644
index 000000000000..16738d306f02
--- /dev/null
+++ b/packages/open-responses/src/responses/__fixtures__/issue-18872-truncated-reasoning.chunks.txt
@@ -0,0 +1,2 @@
+{"type":"response.output_item.added","sequence_number":0,"output_index":0,"item":{"id":"rs_actual_reasoning_id","type":"reasoning","status":"in_progress","summary":[]}}
+{"type":"response.incomplete","sequence_number":1,"response":{"status":"incomplete","incomplete_details":{"reason":"max_output_tokens"},"usage":{"input_tokens":1,"input_tokens_details":{"cached_tokens":0},"output_tokens":1,"output_tokens_details":{"reasoning_tokens":1},"total_tokens":2}}}
diff --git a/packages/open-responses/src/responses/__fixtures__/issue-18872-unknown-incomplete-reason.chunks.txt b/packages/open-responses/src/responses/__fixtures__/issue-18872-unknown-incomplete-reason.chunks.txt
new file mode 100644
index 000000000000..a6defd12a4d3
--- /dev/null
+++ b/packages/open-responses/src/responses/__fixtures__/issue-18872-unknown-incomplete-reason.chunks.txt
@@ -0,0 +1,4 @@
+{"type":"response.output_item.added","sequence_number":0,"output_index":0,"item":{"id":"fc_weather","type":"function_call","status":"in_progress","call_id":"call_weather","name":"get_weather","arguments":""}}
+{"type":"response.function_call_arguments.done","sequence_number":1,"item_id":"fc_weather","output_index":0,"arguments":"{\"location\":\"San Francisco\"}"}
+{"type":"response.output_item.done","sequence_number":2,"output_index":0,"item":{"id":"fc_weather","type":"function_call","status":"completed","call_id":"call_weather","name":"get_weather","arguments":"{\"location\":\"San Francisco\"}"}}
+{"type":"response.incomplete","sequence_number":3,"response":{"status":"incomplete","incomplete_details":{"reason":"server_timeout"},"usage":{"input_tokens":1,"input_tokens_details":{"cached_tokens":0},"output_tokens":1,"output_tokens_details":{"reasoning_tokens":0},"total_tokens":2}}}
diff --git a/packages/open-responses/src/responses/open-responses-language-model.test.ts b/packages/open-responses/src/responses/open-responses-language-model.test.ts
index 960e83c839f1..5657573a2a5d 100644
--- a/packages/open-responses/src/responses/open-responses-language-model.test.ts
+++ b/packages/open-responses/src/responses/open-responses-language-model.test.ts
@@ -706,6 +706,54 @@ describe('OpenResponsesLanguageModel', () => {
       };
     }
 
+    describe('truncated responses', () => {
+      it('should close active reasoning with its actual item id', async () => {
+        prepareChunksFixtureResponse('issue-18872-truncated-reasoning');
+
+        const result = await createModel().doStream({
+          prompt: TEST_PROMPT,
+        });
+        const parts = await convertReadableStreamToArray(result.stream);
+        const reasoningStart = parts.find(
+          part => part.type === 'reasoning-start',
+        );
+        const reasoningEnd = parts.find(part => part.type === 'reasoning-end');
+
+        expect(reasoningStart).toMatchObject({
+          type: 'reasoning-start',
+          id: 'rs_actual_reasoning_id',
+        });
+        expect(reasoningEnd).toMatchObject({
+          type: 'reasoning-end',
+          id: 'rs_actual_reasoning_id',
+        });
+        expect(parts.at(-1)).toMatchObject({
+          type: 'finish',
+          finishReason: {
+            unified: 'length',
+            raw: 'max_output_tokens',
+          },
+        });
+      });
+
+      it('should map an unknown incomplete reason to other even when tool calls exist', async () => {
+        prepareChunksFixtureResponse('issue-18872-unknown-incomplete-reason');
+
+        const result = await createModel().doStream({
+          prompt: TEST_PROMPT,
+        });
+        const parts = await convertReadableStreamToArray(result.stream);
+
+        expect(parts.at(-1)).toMatchObject({
+          type: 'finish',
+          finishReason: {
+            unified: 'other',
+            raw: 'server_timeout',
+          },
+        });
+      });
+    });
+
     describe('basic generation', () => {
       it('should stream content', async () => {
         prepareChunksFixtureResponse('lmstudio-basic.1');