diff --git a/apps/cli/src/repl.ts b/apps/cli/src/repl.ts
index dea742d..88ee6ee 100644
--- a/apps/cli/src/repl.ts
+++ b/apps/cli/src/repl.ts
@@ -358,6 +358,11 @@ export async function startRepl(opts: ReplOpts): Promise<number> {
       ctx.mode = 'default';
       output.write('\n  ▶ Exited plan mode (agent will now execute).\n');
     }
+    // Honor EnterPlanMode tool signal — flip into plan mode (writes blocked).
+    if (result.modeSignal?.enterPlanMode && ctx.mode !== 'plan') {
+      ctx.mode = 'plan';
+      output.write('\n  ◐ Entered plan mode (write tools blocked until you exit).\n');
+    }
     output.write('\n');
     if (result.stopReason === 'error') {
       output.write('  ✕ Error during agent loop. Try again or /status to inspect.\n\n');
diff --git a/packages/core/src/agent.ts b/packages/core/src/agent.ts
index 7ba9bc5..b999040 100644
--- a/packages/core/src/agent.ts
+++ b/packages/core/src/agent.ts
@@ -96,7 +96,7 @@ export interface RunAgentResult {
   /** Reason the loop terminated. */
   stopReason: 'end_turn' | 'max_turns' | 'aborted' | 'error';
   /** Mode-control signals flipped by tools during this run (M3c-rest). */
-  modeSignal?: { exitPlanMode?: boolean };
+  modeSignal?: { exitPlanMode?: boolean; enterPlanMode?: boolean };
 }
 
 const DEFAULT_MAX_TURNS = 16;
@@ -155,9 +155,9 @@ export async function runAgent(opts: RunAgentOptions): Promise<RunAgentResult> {
     if (opts.session) await opts.session.manager.append(opts.session.id, userMsg);
   }
 
-  // modeSignal is mutable — ExitPlanMode flips exitPlanMode = true; the agent
-  // loop owner reads this between turns to switch mode plan → default.
-  const modeSignal: { exitPlanMode?: boolean } = {};
+  // modeSignal is mutable — EnterPlanMode / ExitPlanMode flip these; the agent
+  // loop owner reads them after the run to switch mode (default ⇄ plan).
+  const modeSignal: { exitPlanMode?: boolean; enterPlanMode?: boolean } = {};
   const toolCtx: ToolContext = {
     cwd: opts.cwd,
     signal: opts.signal,
diff --git a/packages/core/src/tools/enter-plan.test.ts b/packages/core/src/tools/enter-plan.test.ts
new file mode 100644
index 0000000..e85d90a
--- /dev/null
+++ b/packages/core/src/tools/enter-plan.test.ts
@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest';
+import { EnterPlanModeTool } from './enter-plan.js';
+
+describe('EnterPlanModeTool', () => {
+  it('flips modeSignal.enterPlanMode and echoes the reason', async () => {
+    const signal: { enterPlanMode?: boolean } = {};
+    const r = await EnterPlanModeTool.execute(
+      { reason: 'the refactor touches many files' },
+      { cwd: '/x', modeSignal: signal },
+    );
+    expect(r.isError).toBeFalsy();
+    expect(signal.enterPlanMode).toBe(true);
+    expect(r.content).toContain('the refactor touches many files');
+    expect((r.data as { enterPlanMode: boolean }).enterPlanMode).toBe(true);
+  });
+
+  it('still succeeds when no modeSignal is passed (best-effort)', async () => {
+    const r = await EnterPlanModeTool.execute({}, { cwd: '/x' });
+    expect(r.isError).toBeFalsy();
+    expect((r.data as { enterPlanMode: boolean }).enterPlanMode).toBe(true);
+  });
+
+  it('uses a generic message when no reason is given', async () => {
+    const r = await EnterPlanModeTool.execute({}, { cwd: '/x' });
+    expect(r.content).toMatch(/Entering plan mode/);
+  });
+});
diff --git a/packages/core/src/tools/enter-plan.ts b/packages/core/src/tools/enter-plan.ts
new file mode 100644
index 0000000..b422237
--- /dev/null
+++ b/packages/core/src/tools/enter-plan.ts
@@ -0,0 +1,41 @@
+// EnterPlanMode tool — signals the host that the agent wants to STOP executing
+// and switch into read-only "plan" mode (present a plan before touching files).
+// Mirror of ExitPlanMode. The agent-loop owner reads modeSignal.enterPlanMode
+// after the run and switches the active mode default → plan.
+// Spec: docs/DEVELOPMENT_PLAN.md §3.8 / §0.1 (parity tool)
+
+import type { ToolContext, ToolHandler, ToolResult } from '../types.js';
+
+interface EnterInput {
+  reason?: string;
+}
+
+export const EnterPlanModeTool: ToolHandler = {
+  name: 'EnterPlanMode',
+  definition: {
+    name: 'EnterPlanMode',
+    description:
+      'Switch into plan mode: stop making changes and instead research + present a plan for approval before executing. Use when a task is ambiguous or risky enough that the user should review the approach first. Write/Edit/Bash become blocked until the user leaves plan mode (or you call ExitPlanMode). Pass `reason` to explain why planning first.',
+    inputSchema: {
+      type: 'object',
+      properties: {
+        reason: {
+          type: 'string',
+          description: 'Why planning first is warranted (shown to the user).',
+        },
+      },
+      required: [],
+    },
+  },
+  async execute(rawInput: Record<string, unknown>, ctx: ToolContext): Promise<ToolResult> {
+    const input = rawInput as unknown as EnterInput;
+    if (ctx.modeSignal) ctx.modeSignal.enterPlanMode = true;
+    const reason = input?.reason?.trim() ?? '';
+    return {
+      content: reason
+        ? `Entering plan mode — ${reason}. I'll research and present a plan before making changes.`
+        : "Entering plan mode — I'll research and present a plan before making changes.",
+      data: { enterPlanMode: true, reason },
+    };
+  },
+};
diff --git a/packages/core/src/tools/registry.ts b/packages/core/src/tools/registry.ts
index 1d9acff..19d7b96 100644
--- a/packages/core/src/tools/registry.ts
+++ b/packages/core/src/tools/registry.ts
@@ -5,6 +5,7 @@ import type { ToolHandler } from '../types.js';
 import { AskUserQuestionTool } from './ask-user.js';
 import { BashTool } from './bash.js';
 import { EditTool } from './edit.js';
+import { EnterPlanModeTool } from './enter-plan.js';
 import { ExitPlanModeTool } from './exit-plan.js';
 import { GlobTool } from './glob.js';
 import { GrepTool } from './grep.js';
@@ -18,7 +19,7 @@ import { WriteTool } from './write.js';
  * Built-in tools shipped by default.
  *   · 6 P0 tools from M1 (Read/Write/Edit/Bash/Grep/Glob)
  *   · 3 M3c-rest tools (TodoWrite/WebFetch/WebSearch)
- *   · 2 agent-control tools (AskUserQuestion/ExitPlanMode)
+ *   · 3 agent-control tools (AskUserQuestion/EnterPlanMode/ExitPlanMode)
  */
 export const BUILTIN_TOOLS: ToolHandler[] = [
   ReadTool,
@@ -31,6 +32,7 @@ export const BUILTIN_TOOLS: ToolHandler[] = [
   WebFetchTool,
   WebSearchTool,
   AskUserQuestionTool,
+  EnterPlanModeTool,
   ExitPlanModeTool,
 ];
 
diff --git a/packages/core/src/types.ts b/packages/core/src/types.ts
index 6793867..5c71257 100644
--- a/packages/core/src/types.ts
+++ b/packages/core/src/types.ts
@@ -127,10 +127,11 @@ export interface ToolContext {
     multiSelect?: boolean;
   }) => Promise<string>;
   /**
-   * Mutable host state that the ExitPlanMode tool flips. The agent loop reads
-   * this between turns and changes its mode accordingly.
+   * Mutable host state that the EnterPlanMode / ExitPlanMode tools flip. The
+   * agent-loop owner reads this after the run and changes the active mode
+   * accordingly (plan ⇄ default).
    */
-  modeSignal?: { exitPlanMode?: boolean };
+  modeSignal?: { exitPlanMode?: boolean; enterPlanMode?: boolean };
 }
 
 export interface ToolResult {