diff --git a/.pi/extensions/pigibrack/README.md b/.pi/extensions/pigibrack/README.md
new file mode 100644
index 0000000..5d75edd
--- /dev/null
+++ b/.pi/extensions/pigibrack/README.md
@@ -0,0 +1,43 @@
+# pigibrack
+
+**pi guile bracket extension** for structural Scheme/Guile editing.
+
+## What it provides
+
+Tools:
+
+- `pigibrack_read_module(path)`
+- `pigibrack_read_form(path, name)`
+- `pigibrack_replace_form(path, name, newSource)`
+- `pigibrack_insert_form(path, anchorName, position, newSource)`
+- `pigibrack_delete_form(path, name)`
+- `pigibrack_check_syntax({ path } | { source })`
+- `pigibrack_eval_expr(expr, module?)`
+
+Command:
+
+- `/pigibrack-status`
+
+## How to load
+
+This extension is project-local at:
+
+- `.pi/extensions/pigibrack/index.ts`
+
+Pi auto-discovers this path. You can also start explicitly:
+
+```bash
+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/`.
+- 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/check_syntax.scm b/.pi/extensions/pigibrack/guile/check_syntax.scm
new file mode 100644
index 0000000..64bd159
--- /dev/null
+++ b/.pi/extensions/pigibrack/guile/check_syntax.scm
@@ -0,0 +1,41 @@
+(use-modules (ice-9 textual-ports))
+
+(define (->string obj)
+  (call-with-output-string
+    (lambda (port)
+      (write obj port))))
+
+(define (read-all path)
+  (call-with-input-file path get-string-all))
+
+(define (read-all-datums source)
+  (let ((port (open-input-string source)))
+    (let loop ()
+      (let ((datum (read port)))
+        (unless (eof-object? datum)
+          (loop))))))
+
+(define (main argv)
+  (if (< (length argv) 2)
+      (begin
+        (display "usage: guile check_syntax.scm <file>\n")
+        (primitive-exit 2))
+      (let* ((path (list-ref argv 1))
+             (ok?
+              (catch #t
+                (lambda ()
+                  (read-all-datums (read-all path))
+                  #t)
+                (lambda (key . args)
+                  (display (symbol->string key))
+                  (display " ")
+                  (display (->string args))
+                  (newline)
+                  #f))))
+        (if ok?
+            (begin
+              (display "OK\n")
+              (primitive-exit 0))
+            (primitive-exit 1)))))
+
+(main (command-line))
diff --git a/.pi/extensions/pigibrack/guile/eval_expr.scm b/.pi/extensions/pigibrack/guile/eval_expr.scm
new file mode 100644
index 0000000..a440dca
--- /dev/null
+++ b/.pi/extensions/pigibrack/guile/eval_expr.scm
@@ -0,0 +1,71 @@
+(use-modules (ice-9 textual-ports)
+             (ice-9 pretty-print))
+
+(define stdout-start "__PIGI_STDOUT_START__")
+(define stdout-end "__PIGI_STDOUT_END__")
+(define result-start "__PIGI_RESULT_START__")
+(define result-end "__PIGI_RESULT_END__")
+
+(define (->string obj)
+  (call-with-output-string
+    (lambda (port)
+      (write obj port))))
+
+(define (read-all path)
+  (call-with-input-file path get-string-all))
+
+(define (parse-module-spec spec)
+  (call-with-input-string spec read))
+
+(define (eval-expression expr-source module-spec)
+  (when module-spec
+    (set-current-module (resolve-interface module-spec)))
+
+  (let* ((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)
+    (values stdout (get-output-string value-port))))
+
+(define (main argv)
+  (if (< (length argv) 2)
+      (begin
+        (display "usage: guile eval_expr.scm <expr-file> [module-spec]\n")
+        (primitive-exit 2))
+      (let* ((expr-path (list-ref argv 1))
+             (module-spec (if (> (length argv) 2)
+                              (parse-module-spec (list-ref argv 2))
+                              #f))
+             (expr-source (read-all expr-path))
+             (ok?
+              (catch #t
+                (lambda ()
+                  (call-with-values
+                      (lambda ()
+                        (eval-expression expr-source module-spec))
+                    (lambda (stdout value)
+                      (display stdout-start)
+                      (newline)
+                      (display stdout)
+                      (display stdout-end)
+                      (newline)
+                      (display result-start)
+                      (newline)
+                      (display value)
+                      (display result-end)
+                      (newline)))
+                  #t)
+                (lambda (key . args)
+                  (display (symbol->string key))
+                  (display " ")
+                  (display (->string args))
+                  (newline)
+                  #f))))
+        (if ok?
+            (primitive-exit 0)
+            (primitive-exit 1)))))
+
+(main (command-line))
diff --git a/.pi/extensions/pigibrack/index.ts b/.pi/extensions/pigibrack/index.ts
new file mode 100644
index 0000000..65987c3
--- /dev/null
+++ b/.pi/extensions/pigibrack/index.ts
@@ -0,0 +1,719 @@
+import { StringEnum, Type } from '@mariozechner/pi-ai';
+import {
+  DEFAULT_MAX_BYTES,
+  DEFAULT_MAX_LINES,
+  formatSize,
+  truncateHead,
+  truncateTail,
+  withFileMutationQueue,
+  type ExtensionAPI,
+} from '@mariozechner/pi-coding-agent';
+import { existsSync } from 'node:fs';
+import { mkdtemp, readFile, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join, resolve } from 'node:path';
+
+const CHECK_SYNTAX_SCRIPT = resolve(__dirname, 'guile/check_syntax.scm');
+const EVAL_EXPR_SCRIPT = resolve(__dirname, 'guile/eval_expr.scm');
+
+const TOOL_DESCRIPTION_SUFFIX = `Tool output is truncated to ${DEFAULT_MAX_LINES} lines or ${formatSize(DEFAULT_MAX_BYTES)}.`;
+
+interface FormSpan {
+  start: number;
+  end: number;
+  line: number;
+  text: string;
+  name?: string;
+}
+
+interface SyntaxCheckResult {
+  valid: boolean;
+  errors: string[];
+}
+
+interface EvalResult {
+  stdout: string;
+  value: string;
+}
+
+function normalizeUserPath(path: string): string {
+  return path.startsWith('@') ? path.slice(1) : path;
+}
+
+function lineNumberAt(source: string, index: number): number {
+  let line = 1;
+  for (let i = 0; i < index && i < source.length; i += 1) {
+    if (source[i] === '\n') line += 1;
+  }
+  return line;
+}
+
+function isWhitespace(ch: string | undefined): boolean {
+  return ch === ' ' || ch === '\n' || ch === '\r' || ch === '\t' || ch === '\f';
+}
+
+function isDelimiter(ch: string | undefined): boolean {
+  if (ch === undefined) return true;
+  return (
+    isWhitespace(ch) ||
+    ch === '(' ||
+    ch === ')' ||
+    ch === '[' ||
+    ch === ']' ||
+    ch === '"' ||
+    ch === ';'
+  );
+}
+
+function parseTopLevelForms(source: string): FormSpan[] {
+  const len = source.length;
+
+  function skipLineComment(i: number): number {
+    let cursor = i;
+    while (cursor < len && source[cursor] !== '\n') cursor += 1;
+    return cursor;
+  }
+
+  function skipBlockComment(i: number): number {
+    let cursor = i + 2;
+    let depth = 1;
+
+    while (cursor < len - 1) {
+      if (source[cursor] === '#' && source[cursor + 1] === '|') {
+        depth += 1;
+        cursor += 2;
+        continue;
+      }
+      if (source[cursor] === '|' && source[cursor + 1] === '#') {
+        depth -= 1;
+        cursor += 2;
+        if (depth === 0) return cursor;
+        continue;
+      }
+      cursor += 1;
+    }
+
+    throw new Error('Unterminated block comment (#| ... |#).');
+  }
+
+  function parseString(i: number): number {
+    let cursor = i + 1;
+    let escaped = false;
+
+    while (cursor < len) {
+      const ch = source[cursor];
+      if (escaped) {
+        escaped = false;
+      } else if (ch === '\\') {
+        escaped = true;
+      } else if (ch === '"') {
+        return cursor + 1;
+      }
+      cursor += 1;
+    }
+
+    throw new Error('Unterminated string literal.');
+  }
+
+  function parseAtom(i: number): number {
+    let cursor = i;
+
+    while (cursor < len) {
+      const ch = source[cursor];
+      if (isDelimiter(ch)) break;
+      if (ch === '#' && (source[cursor + 1] === '|' || source[cursor + 1] === ';')) break;
+      cursor += 1;
+    }
+
+    if (cursor === i) throw new Error(`Unexpected token at index ${i}.`);
+    return cursor;
+  }
+
+  function skipTrivia(i: number): number {
+    let cursor = i;
+
+    while (cursor < len) {
+      const ch = source[cursor];
+
+      if (isWhitespace(ch)) {
+        cursor += 1;
+        continue;
+      }
+
+      if (ch === ';') {
+        cursor = skipLineComment(cursor);
+        continue;
+      }
+
+      if (ch === '#' && source[cursor + 1] === '|') {
+        cursor = skipBlockComment(cursor);
+        continue;
+      }
+
+      if (ch === '#' && source[cursor + 1] === ';') {
+        cursor += 2;
+        cursor = skipTrivia(cursor);
+        cursor = parseDatum(cursor);
+        continue;
+      }
+
+      break;
+    }
+
+    return cursor;
+  }
+
+  function parseList(i: number): number {
+    const open = source[i];
+    const close = open === '(' ? ')' : ']';
+    let cursor = i + 1;
+
+    while (cursor < len) {
+      cursor = skipTrivia(cursor);
+      if (cursor >= len) break;
+
+      if (source[cursor] === close) return cursor + 1;
+      if (source[cursor] === ')' || source[cursor] === ']') {
+        throw new Error(`Mismatched list delimiter near index ${cursor}.`);
+      }
+
+      cursor = parseDatum(cursor);
+    }
+
+    throw new Error('Unterminated list form.');
+  }
+
+  function parseDatum(i: number): number {
+    const cursor = skipTrivia(i);
+    if (cursor >= len) throw new Error('Unexpected end of input.');
+
+    const ch = source[cursor];
+
+    if (ch === '(' || ch === '[') {
+      return parseList(cursor);
+    }
+
+    if (ch === '"') {
+      return parseString(cursor);
+    }
+
+    if (ch === "'" || ch === '`') {
+      return parseDatum(cursor + 1);
+    }
+
+    if (ch === ',') {
+      return parseDatum(source[cursor + 1] === '@' ? cursor + 2 : cursor + 1);
+    }
+
+    if (ch === '#') {
+      if (source[cursor + 1] === '(' || source[cursor + 1] === '[') {
+        return parseList(cursor + 1);
+      }
+
+      let maybeVectorCursor = cursor + 1;
+      while (/[a-zA-Z0-9]/.test(source[maybeVectorCursor] ?? '')) maybeVectorCursor += 1;
+      if (source[maybeVectorCursor] === '(' || source[maybeVectorCursor] === '[') {
+        return parseList(maybeVectorCursor);
+      }
+    }
+
+    return parseAtom(cursor);
+  }
+
+  const forms: FormSpan[] = [];
+  let cursor = skipTrivia(0);
+
+  while (cursor < len) {
+    const start = cursor;
+    const end = parseDatum(cursor);
+    const text = source.slice(start, end);
+
+    forms.push({
+      start,
+      end,
+      line: lineNumberAt(source, start),
+      text,
+      name: extractFormName(text),
+    });
+
+    cursor = skipTrivia(end);
+  }
+
+  return forms;
+}
+
+function extractFormName(formText: string): string | undefined {
+  const trimmed = formText.trim();
+  if (!trimmed.startsWith('(')) return undefined;
+
+  const patterns = [
+    /^\(\s*(?:define|define-public|define\*)\s+\(([\w!$%&*+\-./:<=>?@^~]+(?:[\w!$%&*+\-./:<=>?@^~]|#[^\s()\[\]]*)?)\b/,
+    /^\(\s*(?:define|define-public|define\*)\s+([^\s()\[\]]+)\b/,
+    /^\(\s*(?:define-syntax|define-syntax-rule|define-macro)\s+\(([^\s()\[\]]+)\b/,
+    /^\(\s*(?:define-syntax|define-syntax-rule|define-macro)\s+([^\s()\[\]]+)\b/,
+    /^\(\s*define-record-type\s+([^\s()\[\]]+)\b/,
+    /^\(\s*define-class\s+([^\s()\[\]]+)\b/,
+  ];
+
+  for (const pattern of patterns) {
+    const match = trimmed.match(pattern);
+    if (match?.[1]) return match[1];
+  }
+
+  return undefined;
+}
+
+function oneLine(text: string): string {
+  return text.replace(/\s+/g, ' ').trim();
+}
+
+function collapseForm(formText: string): string {
+  const trimmed = formText.trim();
+  const lines = trimmed.split('\n');
+
+  let output = lines.length <= 5 ? oneLine(trimmed) : `${oneLine(lines[0] ?? trimmed)} ...`;
+
+  if (output.length > 160) {
+    output = `${output.slice(0, 157)}...`;
+  }
+
+  return output;
+}
+
+function uniqueByName(forms: FormSpan[], name: string): FormSpan {
+  const matches = forms.filter((form) => form.name === name);
+
+  if (matches.length === 0) {
+    const available = Array.from(new Set(forms.map((form) => form.name).filter(Boolean))).slice(
+      0,
+      30,
+    ) as string[];
+    const suffix = available.length > 0 ? ` Available names: ${available.join(', ')}` : '';
+    throw new Error(`No top-level form named "${name}" found.${suffix}`);
+  }
+
+  if (matches.length > 1) {
+    const locations = matches.map((match) => `line ${match.line}`).join(', ');
+    throw new Error(
+      `Multiple forms named "${name}" found (${locations}). Please disambiguate first.`,
+    );
+  }
+
+  return matches[0];
+}
+
+function ensureSingleTopLevelForm(source: string): string {
+  const trimmed = source.trim();
+  if (!trimmed) throw new Error('newSource is empty.');
+
+  const forms = parseTopLevelForms(trimmed);
+  if (forms.length !== 1 || forms[0].start !== 0 || forms[0].end !== trimmed.length) {
+    throw new Error('newSource must contain exactly one top-level form.');
+  }
+
+  return trimmed;
+}
+
+function formatTruncated(text: string, mode: 'head' | 'tail' = 'head'): string {
+  const truncation =
+    mode === 'head'
+      ? truncateHead(text, { maxLines: DEFAULT_MAX_LINES, maxBytes: DEFAULT_MAX_BYTES })
+      : truncateTail(text, { maxLines: DEFAULT_MAX_LINES, maxBytes: DEFAULT_MAX_BYTES });
+
+  let output = truncation.content;
+  if (truncation.truncated) {
+    output += `\n\n[Output truncated: ${truncation.outputLines}/${truncation.totalLines} lines, ${formatSize(truncation.outputBytes)}/${formatSize(truncation.totalBytes)}.]`;
+  }
+
+  return output;
+}
+
+function parseEvalOutput(raw: string): EvalResult {
+  const stdoutStart = '__PIGI_STDOUT_START__\n';
+  const stdoutEnd = '__PIGI_STDOUT_END__\n';
+  const resultStart = '__PIGI_RESULT_START__\n';
+  const resultEnd = '__PIGI_RESULT_END__\n';
+
+  const stdoutStartIndex = raw.indexOf(stdoutStart);
+  const stdoutEndIndex = raw.indexOf(stdoutEnd);
+  const resultStartIndex = raw.indexOf(resultStart);
+  const resultEndIndex = raw.indexOf(resultEnd);
+
+  if (
+    stdoutStartIndex === -1 ||
+    stdoutEndIndex === -1 ||
+    resultStartIndex === -1 ||
+    resultEndIndex === -1
+  ) {
+    return { stdout: '', value: raw.trim() };
+  }
+
+  const stdout = raw.slice(stdoutStartIndex + stdoutStart.length, stdoutEndIndex);
+  const value = raw.slice(resultStartIndex + resultStart.length, resultEndIndex);
+
+  return { stdout: stdout.trimEnd(), value: value.trimEnd() };
+}
+
+async function writeTempScheme(source: string): Promise<string> {
+  const dir = await mkdtemp(join(tmpdir(), 'pigibrack-'));
+  const file = join(dir, 'input.scm');
+  await writeFile(file, source, 'utf8');
+  return file;
+}
+
+export default function pigibrackExtension(pi: ExtensionAPI) {
+  async function guileSyntaxCheck(
+    input: { path?: string; source?: string },
+    signal?: AbortSignal,
+  ): Promise<SyntaxCheckResult> {
+    let targetPath: string | undefined;
+
+    if (input.path) {
+      targetPath = input.path;
+    } else if (typeof input.source === 'string') {
+      targetPath = await writeTempScheme(input.source);
+    }
+
+    if (!targetPath) {
+      return { valid: false, errors: ['Either path or source must be provided.'] };
+    }
+
+    const result = await pi.exec('guile', [CHECK_SYNTAX_SCRIPT, targetPath], {
+      signal,
+      timeout: 30_000,
+    });
+
+    if (result.code === 0) {
+      return { valid: true, errors: [] };
+    }
+
+    const message =
+      [result.stderr, result.stdout].filter(Boolean).join('\n').trim() || 'Unknown syntax error';
+    return { valid: false, errors: [message] };
+  }
+
+  async function guileEval(
+    expr: string,
+    moduleSpec: string | undefined,
+    cwd: string,
+    signal?: AbortSignal,
+  ): Promise<EvalResult> {
+    const exprPath = await writeTempScheme(expr);
+    const args = ['-L', cwd, EVAL_EXPR_SCRIPT, exprPath];
+    if (moduleSpec) args.push(moduleSpec);
+
+    const result = await pi.exec('guile', args, {
+      signal,
+      timeout: 30_000,
+      cwd,
+    });
+
+    if (result.code !== 0) {
+      const message =
+        [result.stderr, result.stdout].filter(Boolean).join('\n').trim() ||
+        'Guile evaluation failed.';
+      throw new Error(message);
+    }
+
+    return parseEvalOutput(result.stdout ?? '');
+  }
+
+  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 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}`,
+        `guile available: ${guileVersion.code === 0 ? 'yes' : 'no'}`,
+      ];
+
+      if (guileVersion.code === 0) {
+        const firstLine = (guileVersion.stdout ?? '').split('\n')[0]?.trim();
+        if (firstLine) lines.push(`guile: ${firstLine}`);
+      }
+
+      ctx.ui.notify(lines.join('\n'), scriptsExist && guileVersion.code === 0 ? 'info' : 'warning');
+    },
+  });
+
+  pi.registerTool({
+    name: 'pigibrack_read_module',
+    label: 'pigibrack read module',
+    description: `Read a Scheme file as collapsed top-level forms (name + one-line summary). ${TOOL_DESCRIPTION_SUFFIX}`,
+    promptSnippet: 'Inspect Scheme modules by top-level forms before editing.',
+    promptGuidelines: [
+      'Use pigibrack_read_module before pigibrack_read_form for large Scheme files.',
+    ],
+    parameters: Type.Object({
+      path: Type.String({ description: 'Path to a Scheme source file' }),
+    }),
+    async execute(_toolCallId, params, _signal, _onUpdate, ctx) {
+      const absolutePath = resolve(ctx.cwd, normalizeUserPath(params.path));
+      const source = await readFile(absolutePath, 'utf8');
+      const forms = parseTopLevelForms(source);
+
+      const lines = forms.map((form, index) => {
+        const name = form.name ? ` name=${form.name}` : '';
+        return `${String(index + 1).padStart(3, ' ')}. line ${form.line}${name} :: ${collapseForm(form.text)}`;
+      });
+
+      const output = `Module: ${params.path}\nTop-level forms: ${forms.length}\n\n${lines.join('\n')}`;
+
+      return {
+        content: [{ type: 'text', text: formatTruncated(output, 'head') }],
+        details: {
+          path: params.path,
+          absolutePath,
+          forms: forms.length,
+        },
+      };
+    },
+  });
+
+  pi.registerTool({
+    name: 'pigibrack_read_form',
+    label: 'pigibrack read form',
+    description: `Read one top-level Scheme form by defined name. ${TOOL_DESCRIPTION_SUFFIX}`,
+    promptSnippet: 'Read an individual Scheme top-level form by name.',
+    promptGuidelines: ['Use pigibrack_read_form to fetch a form before replacing it.'],
+    parameters: Type.Object({
+      path: Type.String({ description: 'Path to a Scheme source file' }),
+      name: Type.String({ description: 'Top-level form name (e.g. my-function)' }),
+    }),
+    async execute(_toolCallId, params, _signal, _onUpdate, ctx) {
+      const absolutePath = resolve(ctx.cwd, normalizeUserPath(params.path));
+      const source = await readFile(absolutePath, 'utf8');
+      const forms = parseTopLevelForms(source);
+      const form = uniqueByName(forms, params.name);
+
+      return {
+        content: [{ type: 'text', text: formatTruncated(form.text, 'head') }],
+        details: {
+          path: params.path,
+          absolutePath,
+          name: params.name,
+          line: form.line,
+        },
+      };
+    },
+  });
+
+  pi.registerTool({
+    name: 'pigibrack_replace_form',
+    label: 'pigibrack replace form',
+    description: 'Replace a top-level Scheme form by name, with syntax pre-check via guile reader.',
+    promptSnippet: 'Replace an entire top-level Scheme form by name.',
+    promptGuidelines: [
+      'Always call pigibrack_read_form first, then replace with a complete new form.',
+      'newSource must be exactly one top-level form.',
+    ],
+    parameters: Type.Object({
+      path: Type.String({ description: 'Path to a Scheme source file' }),
+      name: Type.String({ description: 'Top-level form name to replace' }),
+      newSource: Type.String({ description: 'The complete new top-level form source' }),
+    }),
+    async execute(_toolCallId, params, signal, _onUpdate, ctx) {
+      const absolutePath = resolve(ctx.cwd, normalizeUserPath(params.path));
+      const normalizedSource = ensureSingleTopLevelForm(params.newSource);
+
+      const syntax = await guileSyntaxCheck({ source: normalizedSource }, signal);
+      if (!syntax.valid) {
+        throw new Error(`newSource is not valid Scheme: ${syntax.errors.join('; ')}`);
+      }
+
+      return withFileMutationQueue(absolutePath, async () => {
+        const source = await readFile(absolutePath, 'utf8');
+        const forms = parseTopLevelForms(source);
+        const form = uniqueByName(forms, params.name);
+
+        const updated = `${source.slice(0, form.start)}${normalizedSource}${source.slice(form.end)}`;
+        await writeFile(absolutePath, updated, 'utf8');
+
+        return {
+          content: [
+            {
+              type: 'text',
+              text: `Replaced form "${params.name}" at line ${form.line} in ${params.path}.`,
+            },
+          ],
+          details: {
+            path: params.path,
+            absolutePath,
+            name: params.name,
+            line: form.line,
+          },
+        };
+      });
+    },
+  });
+
+  pi.registerTool({
+    name: 'pigibrack_insert_form',
+    label: 'pigibrack insert form',
+    description: 'Insert a new top-level Scheme form before or after an anchor form by name.',
+    promptSnippet: 'Insert a new top-level Scheme form relative to an existing named form.',
+    parameters: Type.Object({
+      path: Type.String({ description: 'Path to a Scheme source file' }),
+      anchorName: Type.String({ description: 'Anchor form name for insertion' }),
+      position: StringEnum(['before', 'after'] as const),
+      newSource: Type.String({ description: 'New top-level form source' }),
+    }),
+    async execute(_toolCallId, params, signal, _onUpdate, ctx) {
+      const absolutePath = resolve(ctx.cwd, normalizeUserPath(params.path));
+      const normalizedSource = ensureSingleTopLevelForm(params.newSource);
+
+      const syntax = await guileSyntaxCheck({ source: normalizedSource }, signal);
+      if (!syntax.valid) {
+        throw new Error(`newSource is not valid Scheme: ${syntax.errors.join('; ')}`);
+      }
+
+      return withFileMutationQueue(absolutePath, async () => {
+        const source = await readFile(absolutePath, 'utf8');
+        const forms = parseTopLevelForms(source);
+        const anchor = uniqueByName(forms, params.anchorName);
+
+        const insertionPoint = params.position === 'before' ? anchor.start : anchor.end;
+
+        const left = source.slice(0, insertionPoint);
+        const right = source.slice(insertionPoint);
+
+        const needsLeadingNewline = left.length > 0 && !left.endsWith('\n');
+        const needsTrailingNewline = right.length > 0 && !right.startsWith('\n');
+
+        const inserted = `${needsLeadingNewline ? '\n' : ''}${normalizedSource}${needsTrailingNewline ? '\n' : ''}`;
+        const updated = `${left}${inserted}${right}`;
+
+        await writeFile(absolutePath, updated, 'utf8');
+
+        return {
+          content: [
+            {
+              type: 'text',
+              text: `Inserted form ${params.position} "${params.anchorName}" (line ${anchor.line}) in ${params.path}.`,
+            },
+          ],
+          details: {
+            path: params.path,
+            absolutePath,
+            anchorName: params.anchorName,
+            position: params.position,
+            line: anchor.line,
+          },
+        };
+      });
+    },
+  });
+
+  pi.registerTool({
+    name: 'pigibrack_delete_form',
+    label: 'pigibrack delete form',
+    description: 'Delete a top-level Scheme form by name.',
+    promptSnippet: 'Delete a top-level Scheme form by its name.',
+    parameters: Type.Object({
+      path: Type.String({ description: 'Path to a Scheme source file' }),
+      name: Type.String({ description: 'Top-level form name to delete' }),
+    }),
+    async execute(_toolCallId, params, _signal, _onUpdate, ctx) {
+      const absolutePath = resolve(ctx.cwd, normalizeUserPath(params.path));
+
+      return withFileMutationQueue(absolutePath, async () => {
+        const source = await readFile(absolutePath, 'utf8');
+        const forms = parseTopLevelForms(source);
+        const form = uniqueByName(forms, params.name);
+
+        let start = form.start;
+        let end = form.end;
+
+        if (source[end] === '\n') end += 1;
+        if (start > 0 && source[start - 1] === '\n' && source[end] === '\n') start -= 1;
+
+        const updated = `${source.slice(0, start)}${source.slice(end)}`;
+        await writeFile(absolutePath, updated, 'utf8');
+
+        return {
+          content: [{ type: 'text', text: `Deleted form "${params.name}" from ${params.path}.` }],
+          details: {
+            path: params.path,
+            absolutePath,
+            name: params.name,
+            line: form.line,
+          },
+        };
+      });
+    },
+  });
+
+  pi.registerTool({
+    name: 'pigibrack_check_syntax',
+    label: 'pigibrack check syntax',
+    description:
+      'Check Scheme syntax via guile reader. Accepts either a file path or inline source.',
+    promptSnippet: 'Validate Scheme syntax before editing or evaluating.',
+    parameters: Type.Object({
+      path: Type.Optional(Type.String({ description: 'Path to a Scheme source file' })),
+      source: Type.Optional(Type.String({ description: 'Inline Scheme source to validate' })),
+    }),
+    async execute(_toolCallId, params, signal, _onUpdate, ctx) {
+      if (!params.path && typeof params.source !== 'string') {
+        throw new Error('Provide either path or source.');
+      }
+
+      if (params.path && typeof params.source === 'string') {
+        throw new Error('Provide path or source, not both.');
+      }
+
+      const resolvedPath = params.path
+        ? resolve(ctx.cwd, normalizeUserPath(params.path))
+        : undefined;
+      const result = await guileSyntaxCheck({ path: resolvedPath, source: params.source }, signal);
+
+      const output = result.valid
+        ? 'Syntax check: valid ✅'
+        : `Syntax check: invalid ❌\n\n${result.errors.map((error, index) => `${index + 1}. ${error}`).join('\n')}`;
+
+      return {
+        content: [{ type: 'text', text: formatTruncated(output, 'head') }],
+        details: {
+          valid: result.valid,
+          errors: result.errors,
+          path: params.path,
+        },
+      };
+    },
+  });
+
+  pi.registerTool({
+    name: 'pigibrack_eval_expr',
+    label: 'pigibrack eval expr',
+    description: `Evaluate a Scheme expression in guile. Optional module is a module spec, e.g. (my module). ${TOOL_DESCRIPTION_SUFFIX}`,
+    promptSnippet: 'Evaluate Scheme expressions in guile for fast feedback.',
+    promptGuidelines: [
+      'Use pigibrack_eval_expr after structural edits to validate behavior quickly.',
+    ],
+    parameters: Type.Object({
+      expr: Type.String({ description: 'A Scheme expression to evaluate' }),
+      module: Type.Optional(Type.String({ description: 'Optional module spec, e.g. (my module)' })),
+    }),
+    async execute(_toolCallId, params, signal, _onUpdate, ctx) {
+      const result = await guileEval(params.expr, params.module, ctx.cwd, signal);
+
+      const outputParts = [] as string[];
+      if (result.stdout.trim()) {
+        outputParts.push(`stdout:\n${result.stdout}`);
+      }
+      outputParts.push(`result:\n${result.value || '<no value>'}`);
+
+      return {
+        content: [{ type: 'text', text: formatTruncated(outputParts.join('\n\n'), 'tail') }],
+        details: {
+          module: params.module,
+          hasStdout: result.stdout.trim().length > 0,
+        },
+      };
+    },
+  });
+}
diff --git a/playground/calc.scm b/playground/calc.scm
new file mode 100644
index 0000000..d7ad42e
--- /dev/null
+++ b/playground/calc.scm
@@ -0,0 +1,10 @@
+(define-module (playground calc)
+  #:export (triple twice greeting))
+
+(define (triple x)
+  (* 3 x))
+
+(define (twice x)
+  (* 2 x))
+
+(define greeting "hello from module")
diff --git a/playground/math.scm b/playground/math.scm
new file mode 100644
index 0000000..09c694a
--- /dev/null
+++ b/playground/math.scm
@@ -0,0 +1,8 @@
+(define (square x)
+  (* x x))
+(define (cube x)
+  (* x x x))
+
+(define (add a b)
+  (+ a b 1))
+