diff --git a/.changeset/fix-reasoning-end-signature.md b/.changeset/fix-reasoning-end-signature.md
new file mode 100644
index 00000000..5e5a3c2c
--- /dev/null
+++ b/.changeset/fix-reasoning-end-signature.md
@@ -0,0 +1,7 @@
+---
+"@openrouter/ai-sdk-provider": patch
+---
+
+fix: include accumulated reasoning_details with signature in reasoning-end stream event
+
+When streaming a text-only response (no tool calls) with reasoning enabled, the reasoning-end event now includes the accumulated reasoning_details (with signature) in providerMetadata. This fixes multi-turn conversation failures with Anthropic models where the signature was lost, causing "Invalid signature in thinking block" errors on subsequent turns.
diff --git a/e2e/issues/issue-394-reasoning-end-signature.test.ts b/e2e/issues/issue-394-reasoning-end-signature.test.ts
new file mode 100644
index 00000000..3b500eb5
--- /dev/null
+++ b/e2e/issues/issue-394-reasoning-end-signature.test.ts
@@ -0,0 +1,132 @@
+/**
+ * Regression test for GitHub PR #394
+ * https://github.com/OpenRouterTeam/ai-sdk-provider/pull/394
+ *
+ * Reported error: Multi-turn conversation failure with Anthropic models when
+ * the first turn is a text-only response (no tool calls) with reasoning enabled.
+ * The reasoning-end stream event was emitted without providerMetadata, causing
+ * the Anthropic signature to be lost. On the next turn, Anthropic rejects with
+ * "Invalid signature in thinking block".
+ */
+import { streamText } from 'ai';
+import { describe, expect, it, vi } from 'vitest';
+import { createOpenRouter } from '@/src';
+
+vi.setConfig({
+  testTimeout: 120_000,
+});
+
+describe('Issue #394: reasoning-end should include accumulated reasoning_details with signature', () => {
+  const provider = createOpenRouter({
+    apiKey: process.env.OPENROUTER_API_KEY,
+    baseUrl: `${process.env.OPENROUTER_API_BASE}/api/v1`,
+  });
+
+  it('should include reasoning_details with signature in reasoning-end providerMetadata for text-only streaming response', async () => {
+    const model = provider('anthropic/claude-sonnet-4');
+
+    const stream = streamText({
+      model,
+      prompt: 'What is 2+2? Answer briefly.',
+      providerOptions: {
+        openrouter: {
+          reasoning: 'enabled',
+        },
+      },
+    });
+
+    let hasReasoningStart = false;
+    let hasReasoningEnd = false;
+    let reasoningEndProviderMetadata: Record<string, unknown> | undefined;
+    let reasoning = '';
+
+    for await (const chunk of stream.fullStream) {
+      if (chunk.type === 'reasoning-start') {
+        hasReasoningStart = true;
+      }
+      if (chunk.type === 'reasoning-delta') {
+        reasoning += chunk.text;
+      }
+      if (chunk.type === 'reasoning-end') {
+        hasReasoningEnd = true;
+        reasoningEndProviderMetadata = chunk.providerMetadata as
+          | Record<string, unknown>
+          | undefined;
+      }
+    }
+
+    expect(hasReasoningStart).toBe(true);
+    expect(hasReasoningEnd).toBe(true);
+    expect(reasoning.length).toBeGreaterThan(0);
+
+    expect(reasoningEndProviderMetadata).toBeDefined();
+
+    const openrouterMeta = reasoningEndProviderMetadata?.openrouter as
+      | Record<string, unknown>
+      | undefined;
+    expect(openrouterMeta).toBeDefined();
+
+    const reasoningDetails = openrouterMeta?.reasoning_details as
+      | Array<Record<string, unknown>>
+      | undefined;
+    expect(reasoningDetails).toBeDefined();
+    expect(reasoningDetails!.length).toBeGreaterThan(0);
+
+    const textDetail = reasoningDetails!.find(
+      (d) => d.type === 'reasoning.text',
+    );
+    expect(textDetail).toBeDefined();
+    expect(textDetail!.signature).toBeDefined();
+    expect(typeof textDetail!.signature).toBe('string');
+    expect((textDetail!.signature as string).length).toBeGreaterThan(0);
+  });
+
+  it('should produce valid reasoning parts for multi-turn continuation', async () => {
+    const model = provider('anthropic/claude-sonnet-4');
+
+    const result = await streamText({
+      model,
+      prompt: 'What is the capital of France? Answer in one word.',
+      providerOptions: {
+        openrouter: {
+          reasoning: 'enabled',
+        },
+      },
+    });
+
+    const response = await result.response;
+    const messages = response.messages;
+
+    expect(messages.length).toBeGreaterThan(0);
+
+    const assistantMessage = messages.find((m) => m.role === 'assistant');
+    expect(assistantMessage).toBeDefined();
+
+    const content = assistantMessage?.content;
+    if (typeof content === 'string') {
+      return;
+    }
+
+    const reasoningParts = content?.filter(
+      (p: { type: string }) => p.type === 'reasoning',
+    );
+
+    if (reasoningParts && reasoningParts.length > 0) {
+      for (const part of reasoningParts) {
+        if ('providerMetadata' in part) {
+          expect(part.providerMetadata).toBeDefined();
+
+          const openrouterMeta = (
+            part as { providerMetadata?: Record<string, unknown> }
+          ).providerMetadata?.openrouter as Record<string, unknown> | undefined;
+          expect(openrouterMeta).toBeDefined();
+
+          const details = openrouterMeta?.reasoning_details as
+            | Array<Record<string, unknown>>
+            | undefined;
+          expect(details).toBeDefined();
+        }
+      }
+    }
+  });
+});
diff --git a/src/chat/index.test.ts b/src/chat/index.test.ts
index fb87b2a2..db8d202f 100644
--- a/src/chat/index.test.ts
+++ b/src/chat/index.test.ts
@@ -2308,6 +2308,104 @@ describe('doStream', () => {
     expect(openrouterMetadata?.annotations?.[0]?.file.hash).toBe('hash1');
     expect(openrouterMetadata?.annotations?.[1]?.file.hash).toBe('hash2');
   });
+
+  it('should include accumulated reasoning_details with signature in reasoning-end providerMetadata for text-only responses', async () => {
+    // This test reproduces the Anthropic multi-turn signature bug:
+    // When streaming a text-only response (no tool calls), the signature arrives
+    // in the LAST reasoning delta. The reasoning-start event gets the FIRST delta's
+    // metadata (no signature). The AI SDK uses reasoning-end's providerMetadata to
+    // update the reasoning part's providerMetadata. So the provider MUST include
+    // the accumulated reasoning_details (with signature) in the reasoning-end event.
+    // Without this fix, the saved reasoning part has no signature, and the next turn
+    // fails with "Invalid signature in thinking block".
+    server.urls['https://openrouter.ai/api/v1/chat/completions']!.response = {
+      type: 'stream-chunks',
+      chunks: [
+        // First chunk: reasoning starts, NO signature yet
+        `data: {"id":"chatcmpl-sig-test","object":"chat.completion.chunk","created":1711357598,"model":"anthropic/claude-opus-4.6",` +
+          `"system_fingerprint":"fp_test","choices":[{"index":0,"delta":{"role":"assistant","content":"",` +
+          `"reasoning_details":[{"type":"${ReasoningDetailType.Text}","text":"Let me think about this","index":0,"format":"anthropic-claude-v1"}]},` +
+          `"logprobs":null,"finish_reason":null}]}\n\n`,
+        // Second chunk: more reasoning text, still no signature
+        `data: {"id":"chatcmpl-sig-test","object":"chat.completion.chunk","created":1711357598,"model":"anthropic/claude-opus-4.6",` +
+          `"system_fingerprint":"fp_test","choices":[{"index":0,"delta":{` +
+          `"reasoning_details":[{"type":"${ReasoningDetailType.Text}","text":" step by step.","index":0,"format":"anthropic-claude-v1"}]},` +
+          `"logprobs":null,"finish_reason":null}]}\n\n`,
+        // Third chunk: last reasoning delta WITH signature
+        `data: {"id":"chatcmpl-sig-test","object":"chat.completion.chunk","created":1711357598,"model":"anthropic/claude-opus-4.6",` +
+          `"system_fingerprint":"fp_test","choices":[{"index":0,"delta":{` +
+          `"reasoning_details":[{"type":"${ReasoningDetailType.Text}","text":" Done.","index":0,"format":"anthropic-claude-v1","signature":"erX9OCAqSEO90HsfvNlBn5J3BQ9cEI/Hg2wHFo5iA8w3L+a"}]},` +
+          `"logprobs":null,"finish_reason":null}]}\n\n`,
+        // Fourth chunk: text content starts (reasoning ends)
+        `data: {"id":"chatcmpl-sig-test","object":"chat.completion.chunk","created":1711357598,"model":"anthropic/claude-opus-4.6",` +
+          `"system_fingerprint":"fp_test","choices":[{"index":0,"delta":{"content":"Hello! How can I help?"},` +
+          `"logprobs":null,"finish_reason":null}]}\n\n`,
+        // Finish chunk
+        `data: {"id":"chatcmpl-sig-test","object":"chat.completion.chunk","created":1711357598,"model":"anthropic/claude-opus-4.6",` +
+          `"system_fingerprint":"fp_test","choices":[{"index":0,"delta":{},` +
+          `"logprobs":null,"finish_reason":"stop"}]}\n\n`,
+        `data: {"id":"chatcmpl-sig-test","object":"chat.completion.chunk","created":1711357598,"model":"anthropic/claude-opus-4.6",` +
+          `"system_fingerprint":"fp_test","choices":[],"usage":{"prompt_tokens":100,"completion_tokens":50,"total_tokens":150}}\n\n`,
+        'data: [DONE]\n\n',
+      ],
+    };
+
+    const { stream } = await model.doStream({
+      prompt: TEST_PROMPT,
+    });
+
+    const elements = await convertReadableStreamToArray(stream);
+
+    // Find reasoning-end event
+    const reasoningEnd = elements.find(
+      (
+        el,
+      ): el is Extract<LanguageModelV3StreamPart, { type: 'reasoning-end' }> =>
+        el.type === 'reasoning-end',
+    );
+
+    expect(reasoningEnd).toBeDefined();
+
+    // The reasoning-end event MUST have providerMetadata with the full accumulated
+    // reasoning_details including the signature from the last delta.
+    // This is critical because the AI SDK updates the reasoning part's providerMetadata
+    // from reasoning-end, and the signature is needed for multi-turn conversations.
+    expect(reasoningEnd?.providerMetadata).toBeDefined();
+
+    const reasoningDetails = (
+      reasoningEnd?.providerMetadata?.openrouter as {
+        reasoning_details: ReasoningDetailUnion[];
+      }
+    )?.reasoning_details;
+
+    expect(reasoningDetails).toBeDefined();
+    expect(reasoningDetails).toHaveLength(1);
+    expect(reasoningDetails[0]).toMatchObject({
+      type: ReasoningDetailType.Text,
+      text: 'Let me think about this step by step. Done.',
+      signature: 'erX9OCAqSEO90HsfvNlBn5J3BQ9cEI/Hg2wHFo5iA8w3L+a',
+      format: 'anthropic-claude-v1',
+    });
+
+    // Also verify that the finish event has the same accumulated data
+    const finishEvent = elements.find(
+      (el): el is Extract<LanguageModelV3StreamPart, { type: 'finish' }> =>
+        el.type === 'finish',
+    );
+
+    const finishReasoningDetails = (
+      finishEvent?.providerMetadata?.openrouter as {
+        reasoning_details: ReasoningDetailUnion[];
+      }
+    )?.reasoning_details;
+
+    expect(finishReasoningDetails).toHaveLength(1);
+    expect(finishReasoningDetails[0]).toMatchObject({
+      type: ReasoningDetailType.Text,
+      text: 'Let me think about this step by step. Done.',
+      signature: 'erX9OCAqSEO90HsfvNlBn5J3BQ9cEI/Hg2wHFo5iA8w3L+a',
+    });
+  });
 });
 
 describe('debug settings', () => {
diff --git a/src/chat/index.ts b/src/chat/index.ts
index a175c168..2d28ddff 100644
--- a/src/chat/index.ts
+++ b/src/chat/index.ts
@@ -822,6 +822,18 @@ export class OpenRouterChatLanguageModel implements LanguageModelV3 {
                 controller.enqueue({
                   type: 'reasoning-end',
                   id: reasoningId || generateId(),
+                  // Include accumulated reasoning_details so the AI SDK can update
+                  // the reasoning part's providerMetadata with the correct signature.
+                  // The signature typically arrives in the last reasoning delta,
+                  // but reasoning-start only carries the first delta's metadata.
+                  providerMetadata:
+                    accumulatedReasoningDetails.length > 0
+                      ? {
+                          openrouter: {
+                            reasoning_details: accumulatedReasoningDetails,
+                          },
+                        }
+                      : undefined,
                 });
                 reasoningStarted = false; // Mark as ended so we don't end it again in flush
               }
@@ -1096,6 +1108,16 @@ export class OpenRouterChatLanguageModel implements LanguageModelV3 {
               controller.enqueue({
                 type: 'reasoning-end',
                 id: reasoningId || generateId(),
+                // Include accumulated reasoning_details so the AI SDK can update
+                // the reasoning part's providerMetadata with the correct signature.
+                providerMetadata:
+                  accumulatedReasoningDetails.length > 0
+                    ? {
+                        openrouter: {
+                          reasoning_details: accumulatedReasoningDetails,
+                        },
+                      }
+                    : undefined,
               });
             }
             if (textStarted) {