diff --git a/packages/core/src/session/runner/to-llm-message.ts b/packages/core/src/session/runner/to-llm-message.ts
index b2b1af5d30f1..948dfc5513be 100644
--- a/packages/core/src/session/runner/to-llm-message.ts
+++ b/packages/core/src/session/runner/to-llm-message.ts
@@ -93,18 +93,41 @@ const assistant = (message: SessionMessage.Assistant, model: Model) => {
     )
     return result ? [call, result] : [call]
   })
-  const meaningful = content.filter((part) => {
+  const retained = content.filter((part) => {
     if (part.type === "text") return part.text !== ""
     if (part.type !== "reasoning") return true
     return part.text !== "" || (part.providerMetadata !== undefined && Object.keys(part.providerMetadata).length > 0)
   })
-  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),
-    )
-    .filter((message) => message !== undefined)
-    .map(Message.tool)
+  // A turn that reasoned before calling a tool must keep the two together: Anthropic
+  // rejects a tool_use that is not preceded by its thinking block. The reasoning part is
+  // dropped above when it carries neither text nor provider state, which is what a failed
+  // step looks like once its provider metadata is stripped — and reasoning text is empty
+  // whenever thinking display is "omitted", the default on current Anthropic models. Drop
+  // the tool parts as well in that case so the replayed turn stays self-consistent rather
+  // than replaying orphaned calls.
+  // Only when replaying to the same model. A model switch lowers reasoning to plain text
+  // on purpose, so the tool calls stay valid and must be kept.
+  const droppedReasoning =
+    sameModel &&
+    message.content.some((item) => item.type === "reasoning") &&
+    !retained.some((part) => part.type === "reasoning")
+  const meaningful = droppedReasoning
+    ? retained.filter((part) => part.type !== "tool-call" && part.type !== "tool-result")
+    : retained
+  const results = droppedReasoning
+    ? []
+    : 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,
+          ),
+        )
+        .filter((message) => message !== undefined)
+        .map(Message.tool)
   if (meaningful.length === 0) return results
   return [
     Message.make({ id: message.id, role: "assistant", content: meaningful, metadata: message.metadata }),
diff --git a/packages/core/test/session-runner-message.test.ts b/packages/core/test/session-runner-message.test.ts
index 5798b665a86a..674617fa87d6 100644
--- a/packages/core/test/session-runner-message.test.ts
+++ b/packages/core/test/session-runner-message.test.ts
@@ -498,4 +498,80 @@ Recent work
       },
     ])
   })
+
+  test("drops tool parts when a failed turn loses its reasoning block", () => {
+    const messages = toLLMMessages(
+      [
+        SessionMessage.Assistant.make({
+          id: id("assistant-thinking-failed"),
+          type: "assistant",
+          agent: "build",
+          model: { id: ModelV2.ID.make("model"), providerID: ProviderV2.ID.make("provider") },
+          content: [
+            // Reasoning text is empty whenever thinking display is "omitted", so once the
+            // failed turn's provider metadata is stripped there is nothing left to replay
+            // and the reasoning part is dropped as empty.
+            SessionMessage.AssistantReasoning.make({
+              type: "reasoning",
+              id: "reasoning-failed",
+              text: "",
+              providerMetadata: { anthropic: { signature: "sig_1" } },
+            }),
+            SessionMessage.AssistantTool.make({
+              type: "tool",
+              id: "orphan-call",
+              name: "read",
+              state: SessionMessage.ToolStatePending.make({ status: "pending", input: '{"path":"README.md"}' }),
+              time: { created },
+            }),
+          ],
+          finish: "error",
+          error: { type: "unknown", message: "Provider turn interrupted" },
+          time: { created, completed: created },
+        }),
+      ],
+      model,
+    )
+
+    // The tool call must not survive on its own: Anthropic rejects a tool_use that is not
+    // preceded by its thinking block.
+    expect(messages).toEqual([])
+  })
+
+  test("keeps tool parts when reasoning survives a failed turn as text", () => {
+    const messages = toLLMMessages(
+      [
+        SessionMessage.Assistant.make({
+          id: id("assistant-text-reasoning-failed"),
+          type: "assistant",
+          agent: "build",
+          model: { id: ModelV2.ID.make("model"), providerID: ProviderV2.ID.make("provider") },
+          content: [
+            SessionMessage.AssistantReasoning.make({
+              type: "reasoning",
+              id: "reasoning-partial",
+              text: "Partial thought",
+              providerMetadata: { openai: { itemId: "rs_failed" } },
+            }),
+            SessionMessage.AssistantTool.make({
+              type: "tool",
+              id: "kept-call",
+              name: "read",
+              state: SessionMessage.ToolStatePending.make({ status: "pending", input: '{"path":"README.md"}' }),
+              time: { created },
+            }),
+          ],
+          finish: "error",
+          error: { type: "unknown", message: "Provider turn interrupted" },
+          time: { created, completed: created },
+        }),
+      ],
+      model,
+    )
+
+    expect(messages[0]?.content).toEqual([
+      { type: "reasoning", text: "Partial thought", providerMetadata: undefined },
+      { type: "tool-call", id: "kept-call", name: "read", input: { path: "README.md" } },
+    ])
+  })
 })