diff --git a/src/agents/pi-embedded-runner.ts b/src/agents/pi-embedded-runner.ts
index c0360c139..2939ed78a 100644
--- a/src/agents/pi-embedded-runner.ts
+++ b/src/agents/pi-embedded-runner.ts
@@ -333,12 +333,15 @@ export async function runEmbeddedPiAgent(params: {
         node: process.version,
         model: `${provider}/${modelId}`,
       };
+      const reasoningTagHint =
+        provider === "lmstudio" || provider === "ollama";
       const systemPrompt = buildSystemPrompt({
         appendPrompt: buildAgentSystemPromptAppend({
           workspaceDir: resolvedWorkspace,
           defaultThinkLevel: params.thinkLevel,
           extraSystemPrompt: params.extraSystemPrompt,
           ownerNumbers: params.ownerNumbers,
+          reasoningTagHint,
           runtimeInfo,
         }),
         contextFiles,
diff --git a/src/agents/system-prompt.test.ts b/src/agents/system-prompt.test.ts
index aa0ddcf51..9d4e5de44 100644
--- a/src/agents/system-prompt.test.ts
+++ b/src/agents/system-prompt.test.ts
@@ -22,4 +22,14 @@ describe("buildAgentSystemPromptAppend", () => {
     expect(prompt).not.toContain("## User Identity");
     expect(prompt).not.toContain("Owner numbers:");
   });
+
+  it("adds reasoning tag hint when enabled", () => {
+    const prompt = buildAgentSystemPromptAppend({
+      workspaceDir: "/tmp/clawd",
+      reasoningTagHint: true,
+    });
+
+    expect(prompt).toContain("## Reasoning Format");
+    expect(prompt).toContain("<think>...</think>");
+  });
 });
diff --git a/src/agents/system-prompt.ts b/src/agents/system-prompt.ts
index 683d77ef2..d8429eadc 100644
--- a/src/agents/system-prompt.ts
+++ b/src/agents/system-prompt.ts
@@ -5,6 +5,7 @@ export function buildAgentSystemPromptAppend(params: {
   defaultThinkLevel?: ThinkLevel;
   extraSystemPrompt?: string;
   ownerNumbers?: string[];
+  reasoningTagHint?: boolean;
   runtimeInfo?: {
     host?: string;
     os?: string;
@@ -26,6 +27,9 @@ export function buildAgentSystemPromptAppend(params: {
     ownerNumbers.length > 0
       ? `Owner numbers: ${ownerNumbers.join(", ")}. Treat messages from these numbers as the user (Peter).`
       : undefined;
+  const reasoningHint = params.reasoningTagHint
+    ? "If you must think, put all reasoning inside <think>...</think> only, and never include analysis outside those tags."
+    : undefined;
   const runtimeInfo = params.runtimeInfo;
   const runtimeLines: string[] = [];
   if (runtimeInfo?.host) runtimeLines.push(`Host: ${runtimeInfo.host}`);
@@ -72,6 +76,9 @@ export function buildAgentSystemPromptAppend(params: {
   if (extraSystemPrompt) {
     lines.push("## Group Chat Context", extraSystemPrompt, "");
   }
+  if (reasoningHint) {
+    lines.push("## Reasoning Format", reasoningHint, "");
+  }
 
   lines.push(
     "## Heartbeats",