diff --git a/apps/desktop/e2e/desktop-preview.spec.ts b/apps/desktop/e2e/desktop-preview.spec.ts
index 491c671..87389d0 100644
--- a/apps/desktop/e2e/desktop-preview.spec.ts
+++ b/apps/desktop/e2e/desktop-preview.spec.ts
@@ -48,6 +48,13 @@ test('resumes a thread and completes an approval-gated protocol turn', async ({
   const approve = page.getByRole('button', { name: /^Approve \(↵\)$/ });
   await expect(approve).toBeVisible();
   await expect(main.getByText(/I’ll update the game safely\./)).toBeVisible();
+
+  // Reasoning arrives as its own channel: collapsed, not mixed into the answer.
+  const reasoning = main.locator('details.reasoning').first();
+  await expect(reasoning).toBeVisible();
+  await expect(reasoning.getByText(/The boss needs a phase field\./)).toBeHidden();
+  await reasoning.getByText('thinking').click();
+  await expect(reasoning.getByText(/The boss needs a phase field\./)).toBeVisible();
   await expect(main.locator('.tool-card').filter({ hasText: 'Edit' }).last()).toBeVisible();
 
   await approve.click();
diff --git a/apps/desktop/src/index.css b/apps/desktop/src/index.css
index a314ad5..6f97def 100644
--- a/apps/desktop/src/index.css
+++ b/apps/desktop/src/index.css
@@ -1973,3 +1973,40 @@ select {
 .ch-text {
   overflow-x: auto;
 }
+
+/* ── Reasoning ───────────────────────────────────────────────────────────
+   A dim, collapsed side channel above the answer: reasoner output is long and
+   is not the response, but dropping it entirely hid the most useful thing
+   DeepSeek's reasoner emits. */
+.reasoning {
+  margin: 0 0 8px;
+  border-left: 2px solid var(--line);
+  padding-left: 10px;
+}
+.reasoning > summary {
+  cursor: pointer;
+  color: var(--text-3);
+  font-size: 11.5px;
+  list-style: none;
+  user-select: none;
+}
+.reasoning > summary::-webkit-details-marker {
+  display: none;
+}
+.reasoning > summary::before {
+  content: '▸ ';
+}
+.reasoning[open] > summary::before {
+  content: '▾ ';
+}
+.reasoning-meta {
+  opacity: 0.7;
+}
+.reasoning-body {
+  margin-top: 6px;
+  color: var(--text-3);
+  font-size: 12px;
+  white-space: pre-wrap;
+  max-height: 320px;
+  overflow-y: auto;
+}
diff --git a/apps/desktop/src/lib/protocol-agent.test.ts b/apps/desktop/src/lib/protocol-agent.test.ts
index 3d394cf..dcfc1ad 100644
--- a/apps/desktop/src/lib/protocol-agent.test.ts
+++ b/apps/desktop/src/lib/protocol-agent.test.ts
@@ -27,6 +27,7 @@ class FakeTransport implements ProtocolTransport {
         structuredToolEvents: true,
         interactiveRequests: true,
         reviewActions: true,
+        reasoningDeltas: true,
         configDiagnostics: true,
         diagnosticExport: true,
         workspaceDiff: true,
diff --git a/apps/desktop/src/lib/protocol-agent.ts b/apps/desktop/src/lib/protocol-agent.ts
index acc6b37..a561268 100644
--- a/apps/desktop/src/lib/protocol-agent.ts
+++ b/apps/desktop/src/lib/protocol-agent.ts
@@ -210,6 +210,14 @@ export class DesktopProtocolAgent {
       case 'item.delta':
         this.emit({ kind: 'event', turnId: event.turnId, type: 'text_delta', text: event.delta });
         break;
+      case 'reasoning.delta':
+        this.emit({
+          kind: 'event',
+          turnId: event.turnId,
+          type: 'thinking_delta',
+          text: event.delta,
+        });
+        break;
       case 'tool.started':
         this.emit({
           kind: 'event',
diff --git a/apps/desktop/src/lib/repl-stream.test.ts b/apps/desktop/src/lib/repl-stream.test.ts
index 6ca8d01..ceb5d7d 100644
--- a/apps/desktop/src/lib/repl-stream.test.ts
+++ b/apps/desktop/src/lib/repl-stream.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from 'vitest';
 import {
+  appendReasoningDelta,
   appendTextDelta,
   appendToolUse,
   attachToolResult,
@@ -281,3 +282,37 @@ describe('threadReviewItems', () => {
     expect(actions).toEqual([{ actionId: 'a1', kind: 'apply' }]);
   });
 });
+
+describe('appendReasoningDelta', () => {
+  it('opens a turn when reasoning arrives before any answer text', () => {
+    const msgs = appendReasoningDelta([], 'first thought');
+    expect(msgs).toHaveLength(1);
+    const a = msgs[0] as AssistantMsg;
+    expect(a.turn.reasoning).toBe('first thought');
+    expect(a.turn.text).toBe('');
+    expect(a.turn.streaming).toBe(true);
+  });
+
+  it('accumulates into the open turn without touching the answer', () => {
+    let msgs = appendReasoningDelta([], 'a');
+    msgs = appendReasoningDelta(msgs, 'b');
+    msgs = appendTextDelta(msgs, 'answer');
+    const a = msgs[0] as AssistantMsg;
+    expect(a.turn.reasoning).toBe('ab');
+    expect(a.turn.text).toBe('answer');
+    expect(msgs).toHaveLength(1);
+  });
+
+  it('starts a new turn when the previous one has finished', () => {
+    let msgs = appendTextDelta([], 'done');
+    msgs = finalizeStreaming(msgs);
+    msgs = appendReasoningDelta(msgs, 'next turn thinking');
+    expect(msgs).toHaveLength(2);
+    expect((msgs[1] as AssistantMsg).turn.reasoning).toBe('next turn thinking');
+  });
+
+  it('does not leak reasoning into the answer text', () => {
+    const msgs = appendTextDelta(appendReasoningDelta([], 'secret plan'), 'visible');
+    expect((msgs[0] as AssistantMsg).turn.text).toBe('visible');
+  });
+});
diff --git a/apps/desktop/src/lib/repl-stream.ts b/apps/desktop/src/lib/repl-stream.ts
index f8cfbb5..f082f04 100644
--- a/apps/desktop/src/lib/repl-stream.ts
+++ b/apps/desktop/src/lib/repl-stream.ts
@@ -20,6 +20,13 @@ export interface ToolInvocation {
 
 export interface AssistantTurn {
   text: string;
+  /**
+   * The model's reasoning for this turn, when it produces any. Kept separate
+   * from `text` so it can be rendered as a distinct, collapsible channel — it
+   * is not the answer, and concatenating it into the answer is how it used to
+   * get dropped instead.
+   */
+  reasoning?: string;
   /** Tool calls interleaved during this turn — rendered as cards after the text. */
   tools: ToolInvocation[];
   streaming: boolean;
@@ -64,6 +71,28 @@ export function appendTextDelta(msgs: Msg[], delta: string): Msg[] {
   return [...msgs, { role: 'assistant', turn: { text: delta, tools: [], streaming: true } }];
 }
 
+/**
+ * Append a reasoning delta to the open assistant turn, opening one if needed.
+ * Reasoning usually arrives *before* any answer text, so this has to be able to
+ * start the turn on its own.
+ */
+export function appendReasoningDelta(msgs: Msg[], delta: string): Msg[] {
+  const idx = lastAssistantIndex(msgs);
+  const target = idx === -1 ? null : (msgs[idx] as AssistantMsg);
+  if (target && target.turn.streaming) {
+    const copy = [...msgs];
+    copy[idx] = {
+      role: 'assistant',
+      turn: { ...target.turn, reasoning: (target.turn.reasoning ?? '') + delta },
+    };
+    return copy;
+  }
+  return [
+    ...msgs,
+    { role: 'assistant', turn: { text: '', reasoning: delta, tools: [], streaming: true } },
+  ];
+}
+
 /** Append a tool invocation to the open assistant turn (same anti-split rule). */
 export function appendToolUse(msgs: Msg[], tool: ToolInvocation): Msg[] {
   const idx = lastAssistantIndex(msgs);
diff --git a/apps/desktop/src/preview-app.tsx b/apps/desktop/src/preview-app.tsx
index 785f5c1..528f0ea 100644
--- a/apps/desktop/src/preview-app.tsx
+++ b/apps/desktop/src/preview-app.tsx
@@ -271,6 +271,7 @@ async function handleProtocolRequest(request: ProtocolRequest): Promise<void> {
           structuredToolEvents: true,
           interactiveRequests: true,
           reviewActions: true,
+          reasoningDeltas: true,
           workspaceDiff: true,
           configDiagnostics: true,
         },
@@ -396,6 +397,13 @@ async function handleProtocolRequest(request: ProtocolRequest): Promise<void> {
       // Emit before the response to exercise the renderer's fast-turn buffer.
       await sendEvent({ type: 'turn.started', threadId: activeThreadId, turn: activeTurn });
       await respond(activeTurn);
+      await sendEvent({
+        type: 'reasoning.delta',
+        threadId: activeThreadId,
+        turnId,
+        itemId: 'assistant-reasoning',
+        delta: 'The boss needs a phase field.\nChecking how spawnBoss reads it.',
+      });
       await sendEvent({
         type: 'item.delta',
         threadId: activeThreadId,
diff --git a/apps/desktop/src/screens/Repl.tsx b/apps/desktop/src/screens/Repl.tsx
index 5280f96..1b7aa41 100644
--- a/apps/desktop/src/screens/Repl.tsx
+++ b/apps/desktop/src/screens/Repl.tsx
@@ -46,6 +46,7 @@ import { projectName } from '../lib/project.js';
 import { useVoice } from '../lib/use-voice.js';
 import { insertTranscript } from '../lib/voice.js';
 import {
+  appendReasoningDelta,
   appendTextDelta,
   appendToolUse,
   attachToolResult,
@@ -366,6 +367,9 @@ export function ReplScreen({
         case 'text_delta':
           setMessages((m) => appendTextDelta(m, e.text ?? ''));
           break;
+        case 'thinking_delta':
+          setMessages((m) => appendReasoningDelta(m, e.text ?? ''));
+          break;
         case 'tool_use': {
           const name = e.name ?? '?';
           const input = e.input ?? {};
@@ -1123,6 +1127,7 @@ function renderMessage(
       <div className="body">
         <div className="author">DeepCode</div>
         <div className="content">
+          {m.turn.reasoning ? <ReasoningBlock text={m.turn.reasoning} /> : null}
           {m.turn.text}
           {m.turn.streaming && isActive && <span className="streaming-cursor" />}
           {m.turn.tools.map((t) => (
@@ -1191,3 +1196,27 @@ function abbreviatePath(p: string): string {
 function truncate(s: string, n: number): string {
   return s.length > n ? s.slice(0, n) + '…\n[truncated]' : s;
 }
+
+/**
+ * The model's reasoning, as a collapsed side channel.
+ *
+ * Collapsed by default: reasoner output is long and is not the answer. Open on
+ * click, and while a turn is still streaming it is often the only thing to look
+ * at, so the summary line reports its length rather than staying silent.
+ */
+function ReasoningBlock({ text }: { text: string }): JSX.Element {
+  const [open, setOpen] = useState(false);
+  const lines = text.split('\n').length;
+  return (
+    <details className="reasoning" open={open} onToggle={(e) => setOpen(e.currentTarget.open)}>
+      <summary>
+        thinking
+        <span className="reasoning-meta">
+          {' · '}
+          {lines} line{lines === 1 ? '' : 's'}
+        </span>
+      </summary>
+      <div className="reasoning-body">{text}</div>
+    </details>
+  );
+}
diff --git a/apps/lsp/src/handler.test.ts b/apps/lsp/src/handler.test.ts
index c8364bd..6e27035 100644
--- a/apps/lsp/src/handler.test.ts
+++ b/apps/lsp/src/handler.test.ts
@@ -22,6 +22,7 @@ const capabilities: InitializeResult = {
     structuredToolEvents: true,
     interactiveRequests: true,
     reviewActions: true,
+    reasoningDeltas: true,
     configDiagnostics: true,
     diagnosticExport: true,
     workspaceDiff: true,
diff --git a/apps/server/src/runtime-executor.ts b/apps/server/src/runtime-executor.ts
index 17720e8..9d47b69 100644
--- a/apps/server/src/runtime-executor.ts
+++ b/apps/server/src/runtime-executor.ts
@@ -134,6 +134,12 @@ export class RuntimeHostExecutor implements TurnExecutor {
             case 'text_delta':
               args.publishDelta(streamingItemId, event.text);
               break;
+            case 'thinking_delta':
+              // Reasoning went nowhere: the protocol carried only
+              // reasoningTokens, so DeepSeek's reasoner produced its most
+              // useful output and every client dropped it.
+              args.publishReasoning(`${streamingItemId}-reasoning`, event.text);
+              break;
             case 'tool_use':
               args.publishToolStarted(event.id, event.name, event.input);
               break;
diff --git a/apps/server/src/server.ts b/apps/server/src/server.ts
index 5dfe261..5113d9d 100644
--- a/apps/server/src/server.ts
+++ b/apps/server/src/server.ts
@@ -36,6 +36,7 @@ export interface TurnExecutionArgs {
   input: Record<string, unknown>;
   signal: AbortSignal;
   publishDelta: (itemId: string, delta: string) => void;
+  publishReasoning: (itemId: string, delta: string) => void;
   publishToolStarted: (itemId: string, name: string, input: Record<string, unknown>) => void;
   publishToolCompleted: (itemId: string, result: { content: string; isError?: boolean }) => void;
   publishUsage: (usage: {
@@ -127,6 +128,7 @@ export class AppServer {
       diagnosticExport: options.diagnosticExport !== undefined,
       workspaceDiff: options.workspaceDiff !== undefined,
       reviewActions: true,
+      reasoningDeltas: true,
     });
   }
 
@@ -382,6 +384,15 @@ export class AppServer {
             delta,
           });
         },
+        publishReasoning: (itemId, delta) => {
+          this.lifecycle.publishReasoning({
+            traceId,
+            threadId: thread.id,
+            turnId: turn.id,
+            itemId,
+            delta,
+          });
+        },
         publishToolStarted: (itemId, name, input) => {
           this.options.onEvent?.({
             type: 'tool.started',
diff --git a/apps/vscode/src/protocol-runtime.test.ts b/apps/vscode/src/protocol-runtime.test.ts
index 21d384f..3450fe7 100644
--- a/apps/vscode/src/protocol-runtime.test.ts
+++ b/apps/vscode/src/protocol-runtime.test.ts
@@ -42,6 +42,7 @@ class FakeClient {
         structuredToolEvents: true,
         interactiveRequests: true,
         reviewActions: true,
+        reasoningDeltas: true,
         configDiagnostics: true,
         diagnosticExport: true,
         workspaceDiff: true,
diff --git a/packages/protocol/src/runtime.test.ts b/packages/protocol/src/runtime.test.ts
index faad833..a5834c7 100644
--- a/packages/protocol/src/runtime.test.ts
+++ b/packages/protocol/src/runtime.test.ts
@@ -36,6 +36,7 @@ describe('ProtocolRuntime', () => {
         structuredToolEvents: true,
         interactiveRequests: true,
         reviewActions: false,
+        reasoningDeltas: false,
         configDiagnostics: false,
         diagnosticExport: false,
         workspaceDiff: false,
diff --git a/packages/protocol/src/runtime.ts b/packages/protocol/src/runtime.ts
index e264d88..a735906 100644
--- a/packages/protocol/src/runtime.ts
+++ b/packages/protocol/src/runtime.ts
@@ -5,6 +5,7 @@ import {
   type DurableProtocolEvent,
   type InitializeResult,
   type ProtocolEvent,
+  type ReasoningDeltaEvent,
   type ThreadSnapshot,
   type TransientDeltaEvent,
   type TurnSnapshot,
@@ -45,6 +46,7 @@ export interface ProtocolRuntimeOptions {
   diagnosticExport?: boolean;
   workspaceDiff?: boolean;
   reviewActions?: boolean;
+  reasoningDeltas?: boolean;
 }
 
 export class ProtocolInvariantError extends Error {
@@ -84,6 +86,7 @@ export class ProtocolRuntime {
         diagnosticExport: this.options.diagnosticExport ?? false,
         workspaceDiff: this.options.workspaceDiff ?? false,
         reviewActions: this.options.reviewActions ?? false,
+        reasoningDeltas: this.options.reasoningDeltas ?? false,
       },
     };
   }
@@ -172,6 +175,11 @@ export class ProtocolRuntime {
     this.emit({ type: 'item.delta', ...event });
   }
 
+  /** Reasoning stream — transient like item.delta, never persisted as an item. */
+  publishReasoning(event: Omit<ReasoningDeltaEvent, 'type'>): void {
+    this.emit({ type: 'reasoning.delta', ...event });
+  }
+
   completeTurn(threadId: string, turnId: string): Promise<TurnSnapshot> {
     return this.finishTurn(threadId, turnId, 'completed');
   }
diff --git a/packages/protocol/src/types.ts b/packages/protocol/src/types.ts
index b4c72bd..65a7aba 100644
--- a/packages/protocol/src/types.ts
+++ b/packages/protocol/src/types.ts
@@ -114,8 +114,26 @@ export interface TransientDeltaEvent {
   delta: string;
 }
 
+/**
+ * A reasoning delta from a model that produces one (DeepSeek's reasoner).
+ *
+ * Separate from `item.delta` rather than a flag on it: reasoning is not the
+ * answer, it is never persisted as a completed item, and a client that doesn't
+ * understand it must be able to drop it without accidentally rendering it as
+ * assistant text. Gated by the `reasoningDeltas` capability.
+ */
+export interface ReasoningDeltaEvent {
+  type: 'reasoning.delta';
+  traceId?: string;
+  threadId: string;
+  turnId: string;
+  itemId: string;
+  delta: string;
+}
+
 export type TransientProtocolEvent =
   | TransientDeltaEvent
+  | ReasoningDeltaEvent
   | ToolStartedEvent
   | ToolCompletedEvent
   | UsageUpdatedEvent
@@ -137,6 +155,8 @@ export interface InitializeResult {
     diagnosticExport: boolean;
     workspaceDiff: boolean;
     reviewActions: boolean;
+    /** Server streams `reasoning.delta` for models that emit reasoning. */
+    reasoningDeltas: boolean;
   };
 }