diff --git a/packages/core/src/session/runner/to-llm-message.ts b/packages/core/src/session/runner/to-llm-message.ts
index b2b1af5d30f1..216ff0bfb595 100644
--- a/packages/core/src/session/runner/to-llm-message.ts
+++ b/packages/core/src/session/runner/to-llm-message.ts
@@ -70,7 +70,12 @@ const toolResult = (tool: SessionMessage.AssistantTool, providerMetadata: Provid
 const assistant = (message: SessionMessage.Assistant, model: Model) => {
   const sameModel =
     String(message.model.providerID) === String(model.provider) && String(message.model.id) === String(model.id)
-  const reuseProviderMetadata = sameModel && message.error === undefined
+  // Reasoning continuation metadata survives failed turns (#38620): replaying a
+  // tool_use part without its preceding signed/redacted thinking block makes
+  // Anthropic reject the request with a 400 once thinking is enabled, so the
+  // reasoning provider state must be reused whenever the turn stays on the same
+  // model. Tool execution metadata from a failed turn is still not trusted.
+  const reuseToolMetadata = sameModel && message.error === undefined
   const content = message.content.flatMap((item): ContentPart[] => {
     if (item.type === "text") return [{ type: "text", text: item.text }]
     if (item.type === "reasoning")
@@ -79,17 +84,17 @@ const assistant = (message: SessionMessage.Assistant, model: Model) => {
             {
               type: "reasoning",
               text: item.text,
-              providerMetadata: reuseProviderMetadata ? item.providerMetadata : undefined,
+              providerMetadata: item.providerMetadata,
             },
           ]
         : item.text.length > 0
           ? [{ type: "text", text: item.text }]
           : []
-    const call = toolCall(item, reuseProviderMetadata ? item.provider?.metadata : undefined)
+    const call = toolCall(item, reuseToolMetadata ? item.provider?.metadata : undefined)
     if (item.provider?.executed !== true) return [call]
     const result = toolResult(
       item,
-      reuseProviderMetadata ? (item.provider.resultMetadata ?? item.provider.metadata) : undefined,
+      reuseToolMetadata ? (item.provider.resultMetadata ?? item.provider.metadata) : undefined,
     )
     return result ? [call, result] : [call]
   })
@@ -101,7 +106,7 @@ const assistant = (message: SessionMessage.Assistant, model: Model) => {
   const results = message.content
     .filter((item): item is SessionMessage.AssistantTool => item.type === "tool" && item.provider?.executed !== true)
     .map((item) =>
-      toolResult(item, reuseProviderMetadata ? (item.provider?.resultMetadata ?? item.provider?.metadata) : undefined),
+      toolResult(item, reuseToolMetadata ? (item.provider?.resultMetadata ?? item.provider?.metadata) : undefined),
     )
     .filter((message) => message !== undefined)
     .map(Message.tool)
diff --git a/packages/core/test/session-runner-message.test.ts b/packages/core/test/session-runner-message.test.ts
index 5798b665a86a..79d6b09fb6c4 100644
--- a/packages/core/test/session-runner-message.test.ts
+++ b/packages/core/test/session-runner-message.test.ts
@@ -327,7 +327,7 @@ Recent work
     ])
   })
 
-  test("drops provider-native continuation metadata from failed assistant turns", () => {
+  test("preserves reasoning continuation metadata on failed same-model assistant turns", () => {
     const messages = toLLMMessages(
       [
         SessionMessage.Assistant.make({
@@ -370,7 +370,11 @@ Recent work
     )
 
     expect(messages[0]?.content).toEqual([
-      { type: "reasoning", text: "Partial thought", providerMetadata: undefined },
+      {
+        type: "reasoning",
+        text: "Partial thought",
+        providerMetadata: { openai: { itemId: "rs_failed", reasoningEncryptedContent: null } },
+      },
       {
         type: "tool-call",
         id: "hosted-failed",
@@ -399,6 +403,131 @@ Recent work
     ])
   })
 
+  test("preserves Anthropic thinking signature on failed assistant turns (#38620)", () => {
+    const messages = toLLMMessages(
+      [
+        SessionMessage.Assistant.make({
+          id: id("assistant-failed-anthropic"),
+          type: "assistant",
+          agent: "build",
+          model: { id: ModelV2.ID.make("model"), providerID: ProviderV2.ID.make("provider") },
+          content: [
+            SessionMessage.AssistantReasoning.make({
+              type: "reasoning",
+              id: "reasoning-failed-anthropic",
+              text: "Signed thought",
+              providerMetadata: { anthropic: { signature: "sig_failed" } },
+            }),
+            SessionMessage.AssistantTool.make({
+              type: "tool",
+              id: "hosted-failed-anthropic",
+              name: "web_search",
+              provider: {
+                executed: true,
+                metadata: { anthropic: { signature: "call_sig_failed" } },
+              },
+              state: SessionMessage.ToolStateError.make({
+                status: "error",
+                input: { query: "Effect" },
+                error: { type: "unknown", message: "Provider turn interrupted" },
+                content: [],
+                structured: {},
+              }),
+              time: { created, completed: created },
+            }),
+          ],
+          finish: "error",
+          error: { type: "unknown", message: "Provider turn interrupted" },
+          time: { created, completed: created },
+        }),
+      ],
+      model,
+    )
+
+    // The replayed tool_use must be preceded by its signed thinking block or
+    // Anthropic rejects the request with a 400 once thinking is enabled.
+    expect(messages[0]?.content).toEqual([
+      {
+        type: "reasoning",
+        text: "Signed thought",
+        providerMetadata: { anthropic: { signature: "sig_failed" } },
+      },
+      {
+        type: "tool-call",
+        id: "hosted-failed-anthropic",
+        name: "web_search",
+        input: { query: "Effect" },
+        providerExecuted: true,
+        providerMetadata: undefined,
+      },
+      {
+        type: "tool-result",
+        id: "hosted-failed-anthropic",
+        name: "web_search",
+        result: {
+          type: "error",
+          value: {
+            error: { type: "unknown", message: "Provider turn interrupted" },
+            content: [],
+            structured: {},
+          },
+        },
+        providerExecuted: true,
+        cache: undefined,
+        metadata: undefined,
+        providerMetadata: undefined,
+      },
+    ])
+  })
+
+  test("preserves redacted thinking data on failed assistant turns (#38620)", () => {
+    const messages = toLLMMessages(
+      [
+        SessionMessage.Assistant.make({
+          id: id("assistant-failed-redacted"),
+          type: "assistant",
+          agent: "build",
+          model: { id: ModelV2.ID.make("model"), providerID: ProviderV2.ID.make("provider") },
+          content: [
+            SessionMessage.AssistantReasoning.make({
+              type: "reasoning",
+              id: "reasoning-failed-redacted",
+              text: "",
+              providerMetadata: { anthropic: { redactedData: "redacted-blob-failed" } },
+            }),
+            SessionMessage.AssistantTool.make({
+              type: "tool",
+              id: "hosted-failed-redacted",
+              name: "web_search",
+              provider: {
+                executed: true,
+                metadata: {},
+              },
+              state: SessionMessage.ToolStateError.make({
+                status: "error",
+                input: { query: "Effect" },
+                error: { type: "unknown", message: "Provider turn interrupted" },
+                content: [],
+                structured: {},
+              }),
+              time: { created, completed: created },
+            }),
+          ],
+          finish: "error",
+          error: { type: "unknown", message: "Provider turn interrupted" },
+          time: { created, completed: created },
+        }),
+      ],
+      model,
+    )
+
+    expect(messages[0]?.content[0]).toEqual({
+      type: "reasoning",
+      text: "",
+      providerMetadata: { anthropic: { redactedData: "redacted-blob-failed" } },
+    })
+  })
+
   test("drops provider-native continuation metadata after a model switch", () => {
     const messages = toLLMMessages(
       [