diff --git a/.pi/extensions/pigibrack/README.md b/.pi/extensions/pigibrack/README.md
index 5d75edd..7c05d55 100644
--- a/.pi/extensions/pigibrack/README.md
+++ b/.pi/extensions/pigibrack/README.md
@@ -14,9 +14,10 @@ Tools:
 - `pigibrack_check_syntax({ path } | { source })`
 - `pigibrack_eval_expr(expr, module?)`
 
-Command:
+Commands:
 
 - `/pigibrack-status`
+- `/pigibrack-repl-reset`
 
 ## How to load
 
@@ -33,11 +34,11 @@ pi -e ./.pi/extensions/pigibrack/index.ts
 ## Notes
 
 - Structural operations are top-level form by **name**.
-- Syntax checks and eval run through **guile** scripts in `guile/`.
+- Syntax checks run through **guile** reader scripts in `guile/`.
+- Eval uses a persistent Guile REPL sidecar (`repl_sidecar.scm`) with fallback to one-shot eval if sidecar startup fails.
 - File mutations use pi's `withFileMutationQueue()` to avoid race conditions in parallel tool mode.
 - Output is truncated to avoid context blowups.
 
 ## Current limitations
 
-- `pigibrack_eval_expr` currently runs a fresh Guile process per call (not yet a persistent REPL session).
 - Name extraction targets common `define*` patterns; unusual macro forms may be unnamed by the tool.
diff --git a/.pi/extensions/pigibrack/guile/repl_sidecar.scm b/.pi/extensions/pigibrack/guile/repl_sidecar.scm
new file mode 100644
index 0000000..1a020eb
--- /dev/null
+++ b/.pi/extensions/pigibrack/guile/repl_sidecar.scm
@@ -0,0 +1,104 @@
+(use-modules (ice-9 pretty-print)
+             (ice-9 rdelim)
+             (ice-9 textual-ports))
+
+(define (->string obj)
+  (call-with-output-string
+    (lambda (port)
+      (write obj port))))
+
+(define (read-text path)
+  (call-with-input-file path get-string-all))
+
+(define (write-text path text)
+  (call-with-output-file path
+    (lambda (port)
+      (display text port))))
+
+(define (split-tabs line)
+  (let ((n (string-length line)))
+    (let loop ((i 0) (start 0) (parts '()))
+      (if (= i n)
+          (reverse (cons (substring line start i) parts))
+          (if (char=? (string-ref line i) #\tab)
+              (loop (+ i 1) (+ i 1) (cons (substring line start i) parts))
+              (loop (+ i 1) start parts))))))
+
+(define (module-spec-from-string raw)
+  (if (or (string=? raw "-") (string-null? raw))
+      #f
+      (call-with-input-string raw read)))
+
+(define (reply text)
+  (display text)
+  (newline)
+  (force-output))
+
+(define (handle-eval parts)
+  (if (< (length parts) 6)
+      (reply "DONE\tERR")
+      (let* ((expr-path (list-ref parts 1))
+             (module-raw (list-ref parts 2))
+             (stdout-path (list-ref parts 3))
+             (value-path (list-ref parts 4))
+             (error-path (list-ref parts 5)))
+        (catch #t
+          (lambda ()
+            (let ((module-spec (module-spec-from-string module-raw)))
+              (when module-spec
+                (set-current-module (resolve-interface module-spec))))
+
+            (let* ((expr-source (read-text expr-path))
+                   (expr (call-with-input-string expr-source read))
+                   (stdout-port (open-output-string))
+                   (value (parameterize ((current-output-port stdout-port))
+                            (eval expr (current-module))))
+                   (stdout (get-output-string stdout-port))
+                   (value-port (open-output-string)))
+              (pretty-print value value-port)
+              (write-text stdout-path stdout)
+              (write-text value-path (get-output-string value-port))
+              (write-text error-path "")
+              (reply "DONE\tOK")))
+          (lambda (key . args)
+            (write-text stdout-path "")
+            (write-text value-path "")
+            (write-text error-path (string-append (symbol->string key) " " (->string args) "\n"))
+            (reply "DONE\tERR"))))))
+
+(define (handle-reset)
+  (set-current-module (resolve-module '(guile-user) #:ensure #t))
+  (reply "DONE\tOK"))
+
+(define (dispatch line)
+  (let* ((parts (split-tabs line))
+         (command (if (null? parts) "" (car parts))))
+    (cond
+      ((string=? command "EVAL")
+       (handle-eval parts)
+       #t)
+      ((string=? command "RESET")
+       (handle-reset)
+       #t)
+      ((string=? command "PING")
+       (reply "PONG")
+       #t)
+      ((string=? command "QUIT")
+       (reply "BYE")
+       #f)
+      (else
+       (reply "DONE\tERR")
+       #t))))
+
+(define (main)
+  (set-current-module (resolve-module '(guile-user) #:ensure #t))
+  (reply "READY")
+
+  (let loop ()
+    (let ((line (read-line)))
+      (if (eof-object? line)
+          (primitive-exit 0)
+          (when (dispatch line)
+            (loop))))))
+
+(main)
diff --git a/.pi/extensions/pigibrack/index.ts b/.pi/extensions/pigibrack/index.ts
index 65987c3..9382c7f 100644
--- a/.pi/extensions/pigibrack/index.ts
+++ b/.pi/extensions/pigibrack/index.ts
@@ -8,13 +8,16 @@ import {
   withFileMutationQueue,
   type ExtensionAPI,
 } from '@mariozechner/pi-coding-agent';
+import { spawn, type ChildProcessWithoutNullStreams } from 'node:child_process';
 import { existsSync } from 'node:fs';
 import { mkdtemp, readFile, writeFile } from 'node:fs/promises';
 import { tmpdir } from 'node:os';
 import { join, resolve } from 'node:path';
+import { createInterface } from 'node:readline';
 
 const CHECK_SYNTAX_SCRIPT = resolve(__dirname, 'guile/check_syntax.scm');
 const EVAL_EXPR_SCRIPT = resolve(__dirname, 'guile/eval_expr.scm');
+const REPL_SIDECAR_SCRIPT = resolve(__dirname, 'guile/repl_sidecar.scm');
 
 const TOOL_DESCRIPTION_SUFFIX = `Tool output is truncated to ${DEFAULT_MAX_LINES} lines or ${formatSize(DEFAULT_MAX_BYTES)}.`;
 
@@ -36,6 +39,21 @@ interface EvalResult {
   value: string;
 }
 
+interface SidecarWaiter {
+  resolve: (line: string) => void;
+  reject: (error: Error) => void;
+}
+
+interface ReplSidecar {
+  process: ChildProcessWithoutNullStreams;
+  cwd: string;
+  lines: string[];
+  waiters: SidecarWaiter[];
+  queue: Promise<void>;
+  stderrTail: string[];
+  closedError?: Error;
+}
+
 function normalizeUserPath(path: string): string {
   return path.startsWith('@') ? path.slice(1) : path;
 }
@@ -362,6 +380,213 @@ async function writeTempScheme(source: string): Promise<string> {
 }
 
 export default function pigibrackExtension(pi: ExtensionAPI) {
+  let replSidecar: ReplSidecar | undefined;
+
+  function closeSidecar(sidecar: ReplSidecar, error: Error) {
+    if (sidecar.closedError) return;
+
+    sidecar.closedError = error;
+    for (const waiter of sidecar.waiters) {
+      waiter.reject(error);
+    }
+    sidecar.waiters = [];
+
+    if (replSidecar === sidecar) {
+      replSidecar = undefined;
+    }
+  }
+
+  function readSidecarLine(sidecar: ReplSidecar): Promise<string> {
+    if (sidecar.lines.length > 0) {
+      return Promise.resolve(sidecar.lines.shift() as string);
+    }
+
+    if (sidecar.closedError) {
+      return Promise.reject(sidecar.closedError);
+    }
+
+    return new Promise<string>((resolve, reject) => {
+      sidecar.waiters.push({ resolve, reject });
+    });
+  }
+
+  async function waitForSidecarLine(
+    sidecar: ReplSidecar,
+    timeoutMs: number,
+    signal?: AbortSignal,
+  ): Promise<string> {
+    const linePromise = readSidecarLine(sidecar);
+
+    return await new Promise<string>((resolve, reject) => {
+      let done = false;
+
+      const finish = (fn: () => void) => {
+        if (done) return;
+        done = true;
+        clearTimeout(timeout);
+        if (signal) {
+          signal.removeEventListener('abort', onAbort);
+        }
+        fn();
+      };
+
+      const timeout = setTimeout(() => {
+        finish(() =>
+          reject(new Error(`Timed out waiting for sidecar response after ${timeoutMs}ms.`)),
+        );
+      }, timeoutMs);
+
+      const onAbort = () => {
+        finish(() => reject(new Error('Operation cancelled.')));
+      };
+
+      if (signal) {
+        if (signal.aborted) {
+          finish(() => reject(new Error('Operation cancelled.')));
+          return;
+        }
+        signal.addEventListener('abort', onAbort, { once: true });
+      }
+
+      linePromise.then(
+        (line) => finish(() => resolve(line)),
+        (error) => finish(() => reject(error)),
+      );
+    });
+  }
+
+  function enqueueSidecar<T>(sidecar: ReplSidecar, task: () => Promise<T>): Promise<T> {
+    const run = sidecar.queue.then(task, task);
+    sidecar.queue = run.then(
+      () => undefined,
+      () => undefined,
+    );
+    return run;
+  }
+
+  function createReplSidecar(cwd: string): ReplSidecar {
+    const process = spawn('guile', ['-L', cwd, REPL_SIDECAR_SCRIPT], {
+      cwd,
+      stdio: ['pipe', 'pipe', 'pipe'],
+    });
+
+    const sidecar: ReplSidecar = {
+      process,
+      cwd,
+      lines: [],
+      waiters: [],
+      queue: Promise.resolve(),
+      stderrTail: [],
+    };
+
+    const stdoutReader = createInterface({ input: process.stdout });
+    stdoutReader.on('line', (line) => {
+      if (sidecar.waiters.length > 0) {
+        const waiter = sidecar.waiters.shift() as SidecarWaiter;
+        waiter.resolve(line);
+      } else {
+        sidecar.lines.push(line);
+      }
+    });
+
+    process.stderr.on('data', (chunk) => {
+      const text = String(chunk);
+      for (const line of text.split('\n')) {
+        const trimmed = line.trim();
+        if (!trimmed) continue;
+        sidecar.stderrTail.push(trimmed);
+        if (sidecar.stderrTail.length > 20) {
+          sidecar.stderrTail.shift();
+        }
+      }
+    });
+
+    process.on('error', (error) => {
+      closeSidecar(sidecar, error instanceof Error ? error : new Error(String(error)));
+    });
+
+    process.on('close', (code, signal) => {
+      const stderrSummary =
+        sidecar.stderrTail.length > 0 ? ` stderr: ${sidecar.stderrTail.join(' | ')}` : '';
+      closeSidecar(
+        sidecar,
+        new Error(
+          `Guile sidecar exited (code=${String(code)}, signal=${String(signal)}).${stderrSummary}`,
+        ),
+      );
+    });
+
+    return sidecar;
+  }
+
+  async function stopReplSidecar() {
+    if (!replSidecar) return;
+
+    const sidecar = replSidecar;
+    replSidecar = undefined;
+
+    if (sidecar.closedError) {
+      return;
+    }
+
+    try {
+      sidecar.process.stdin.write('QUIT\n');
+    } catch {
+      // ignore
+    }
+
+    await new Promise<void>((resolve) => {
+      const timer = setTimeout(() => {
+        if (!sidecar.process.killed) {
+          sidecar.process.kill('SIGTERM');
+        }
+      }, 500);
+
+      sidecar.process.once('close', () => {
+        clearTimeout(timer);
+        resolve();
+      });
+    });
+  }
+
+  async function ensureReplSidecar(cwd: string, signal?: AbortSignal): Promise<ReplSidecar> {
+    if (replSidecar && !replSidecar.closedError && replSidecar.cwd !== cwd) {
+      await stopReplSidecar();
+    }
+
+    if (!replSidecar || replSidecar.closedError) {
+      const sidecar = createReplSidecar(cwd);
+      const ready = await waitForSidecarLine(sidecar, 5_000, signal);
+      if (ready !== 'READY') {
+        closeSidecar(sidecar, new Error(`Unexpected sidecar handshake: ${ready}`));
+        throw new Error(`Failed to start pigibrack REPL sidecar: ${ready}`);
+      }
+      replSidecar = sidecar;
+    }
+
+    return replSidecar;
+  }
+
+  function sanitizeModuleSpec(moduleSpec: string | undefined): string {
+    if (!moduleSpec) return '-';
+    return moduleSpec.replace(/[\t\n\r]/g, ' ').trim() || '-';
+  }
+
+  async function sidecarCommand(
+    cwd: string,
+    command: string,
+    signal?: AbortSignal,
+  ): Promise<string> {
+    const sidecar = await ensureReplSidecar(cwd, signal);
+
+    return enqueueSidecar(sidecar, async () => {
+      if (sidecar.closedError) throw sidecar.closedError;
+
+      sidecar.process.stdin.write(`${command}\n`);
+      return await waitForSidecarLine(sidecar, 30_000, signal);
+    });
+  }
+
   async function guileSyntaxCheck(
     input: { path?: string; source?: string },
     signal?: AbortSignal,
@@ -392,7 +617,7 @@ export default function pigibrackExtension(pi: ExtensionAPI) {
     return { valid: false, errors: [message] };
   }
 
-  async function guileEval(
+  async function guileEvalDirect(
     expr: string,
     moduleSpec: string | undefined,
     cwd: string,
@@ -418,19 +643,72 @@ export default function pigibrackExtension(pi: ExtensionAPI) {
     return parseEvalOutput(result.stdout ?? '');
   }
 
+  async function guileEval(
+    expr: string,
+    moduleSpec: string | undefined,
+    cwd: string,
+    signal?: AbortSignal,
+  ): Promise<EvalResult> {
+    const dir = await mkdtemp(join(tmpdir(), 'pigibrack-repl-'));
+    const exprPath = join(dir, 'expr.scm');
+    const stdoutPath = join(dir, 'stdout.txt');
+    const valuePath = join(dir, 'value.txt');
+    const errorPath = join(dir, 'error.txt');
+
+    await writeFile(exprPath, expr, 'utf8');
+
+    const command = [
+      'EVAL',
+      exprPath,
+      sanitizeModuleSpec(moduleSpec),
+      stdoutPath,
+      valuePath,
+      errorPath,
+    ].join('\t');
+
+    try {
+      const response = await sidecarCommand(cwd, command, signal);
+      const stdout = (await readFile(stdoutPath, 'utf8').catch(() => '')).trimEnd();
+      const value = (await readFile(valuePath, 'utf8').catch(() => '')).trimEnd();
+      const error = (await readFile(errorPath, 'utf8').catch(() => '')).trim();
+
+      if (response === 'DONE\tOK') {
+        return { stdout, value };
+      }
+
+      if (response === 'DONE\tERR') {
+        throw new Error(error || 'Guile sidecar evaluation failed.');
+      }
+
+      throw new Error(`Unexpected sidecar response: ${response}`);
+    } catch (error) {
+      // Fallback to non-persistent eval script so eval still works if sidecar is unavailable.
+      return await guileEvalDirect(expr, moduleSpec, cwd, signal);
+    }
+  }
+
   pi.registerCommand('pigibrack-status', {
     description: 'Show pigibrack extension status and guile availability',
     handler: async (_args, ctx) => {
-      const scriptsExist = existsSync(CHECK_SYNTAX_SCRIPT) && existsSync(EVAL_EXPR_SCRIPT);
+      const scriptsExist =
+        existsSync(CHECK_SYNTAX_SCRIPT) &&
+        existsSync(EVAL_EXPR_SCRIPT) &&
+        existsSync(REPL_SIDECAR_SCRIPT);
       const guileVersion = await pi.exec('guile', ['--version'], { timeout: 5_000 });
 
       const lines = [
         `scripts: ${scriptsExist ? 'ok' : 'missing'}`,
         `check_syntax.scm: ${CHECK_SYNTAX_SCRIPT}`,
         `eval_expr.scm: ${EVAL_EXPR_SCRIPT}`,
+        `repl_sidecar.scm: ${REPL_SIDECAR_SCRIPT}`,
         `guile available: ${guileVersion.code === 0 ? 'yes' : 'no'}`,
+        `sidecar running: ${replSidecar && !replSidecar.closedError ? 'yes' : 'no'}`,
       ];
 
+      if (replSidecar && !replSidecar.closedError) {
+        lines.push(`sidecar cwd: ${replSidecar.cwd}`);
+      }
+
       if (guileVersion.code === 0) {
         const firstLine = (guileVersion.stdout ?? '').split('\n')[0]?.trim();
         if (firstLine) lines.push(`guile: ${firstLine}`);
@@ -440,6 +718,21 @@ export default function pigibrackExtension(pi: ExtensionAPI) {
     },
   });
 
+  pi.registerCommand('pigibrack-repl-reset', {
+    description: 'Reset the pigibrack persistent guile REPL sidecar',
+    handler: async (_args, ctx) => {
+      const response = await sidecarCommand(ctx.cwd, 'RESET');
+      if (response !== 'DONE\tOK') {
+        throw new Error(`Unexpected sidecar response: ${response}`);
+      }
+      ctx.ui.notify('pigibrack REPL sidecar reset.', 'info');
+    },
+  });
+
+  pi.on('session_shutdown', async () => {
+    await stopReplSidecar();
+  });
+
   pi.registerTool({
     name: 'pigibrack_read_module',
     label: 'pigibrack read module',