Emacs Integration

The mcp__emacs__emacs_eval tool evaluates Emacs Lisp in the running Emacs instance.

When to Use Emacs vs CLI Tools

Use Emacs (emacs_eval) for:

•Semantic operations (LSP-aware):

elisp

(eglot-rename "new_name")          ; LSP knows scope, not just text
(xref-find-references "func")      ; Understands imports, aliases
(flymake-diagnostics)              ; Type errors, not just syntax
(project-find-file)                ; Project-aware file finding

•Elisp codebase exploration (STRONGLY PREFERRED over grep/glob):

elisp

;; Discover symbols matching a pattern
(apropos-internal "agent-shell.*hook" 'boundp)  ; Find variables
(apropos-internal "^gptel-make" 'fboundp)       ; Find functions

;; Get function/variable documentation instantly
(describe-function 'agent-shell-mcp-server-start)
(describe-variable 'agent-shell-mode-hook)

;; Check current runtime state
(agent-shell-mcp-server-running-p)              ; Live state check
(symbol-value 'some-variable)                   ; Current value

;; Find files
(directory-files-recursively "~/.emacs.d" "\\.el$")

Use CLI tools (Bash/Grep) only for:

bash

grep -r "TODO" --include="*.py" .  # Bulk text search in non-Elisp
sed -i 's/old/new/g' *.py          # Global text replace
find . -name "*.test.js"           ; File finding by pattern

Rule of thumb: For Emacs/Elisp codebases, ALWAYS use emacs_eval first. It's more token-efficient (returns only what you need), provides live runtime state, and understands Elisp semantics. Only fall back to grep/glob for non-Elisp code or bulk text operations.

Why Elisp is Powerful for Agents

•No access control - Everything is public, agent can modify anything
•Advice system - Wrap any function without modifying source: (advice-add 'fn :before #'my-fn)
•Instant eval - No compilation step, immediate feedback
•Self-documenting - (describe-function 'fn) at runtime
•Hooks everywhere - Event-driven observation and intervention

Documentation Discovery

Query Emacs directly for any Elisp documentation:

elisp

(describe-function 'function-name)
(describe-variable 'variable-name)
(apropos "search-term")

Current Context

The Claude Code session runs in an agent-shell-mode buffer. Get context:

elisp

major-mode                        ; => agent-shell-mode
(buffer-name)                     ; => "Claude Code Agent @ project-name"
(agent-shell--current-shell)      ; Get the shell object

Self-Prompting with Timers

Set up a timer to automatically queue a prompt after idle time:

elisp

(defvar my-agent-idle-timer nil)

(defun my-agent-idle-prompt ()
  "Queue a nudge to the agent after idle timeout."
  (when-let ((shell-buf (get-buffer "Claude Code Agent @ .emacs.d")))
    (with-current-buffer shell-buf
      ;; Use enqueue - agent-shell-insert errors if agent is busy
      (agent-shell--enqueue-request
       :prompt "Continue working on the current task."))))

;; Start: prompt after 15 minutes (900 seconds) of no activity
(setq my-agent-idle-timer
      (run-with-idle-timer 900 t #'my-agent-idle-prompt))

;; Stop the timer
(when my-agent-idle-timer
  (cancel-timer my-agent-idle-timer)
  (setq my-agent-idle-timer nil))

Edge case: agent-shell-insert with :submit t returns "Busy" or "Not yet supported" errors while the agent is processing. Use agent-shell--enqueue-request instead - it queues the request to run after the current task completes.

Queueing Requests

Queue prompts that execute when the agent is no longer busy:

elisp

;; Queue a request (runs when current task completes)
(agent-shell--enqueue-request :prompt "Next task to do")

;; Check pending queue
(with-current-buffer (agent-shell--current-shell)
  (alist-get :pending-requests agent-shell--state))

Response Size Limits

Large return values may exceed the MCP response limit. Strategies:

elisp

;; Truncate long strings
(substring (buffer-string) 0 (min 10000 (length (buffer-string))))

;; Return counts instead of full lists
(length (buffer-list))

;; Filter before returning
(seq-take (buffer-list) 10)

;; Write to file, return path
(with-temp-file "/tmp/output.txt"
  (insert large-content))
"/tmp/output.txt"  ; Return just the path

Buffer Context Gotcha

Evaluations run in the agent-shell buffer by default. To operate on other buffers:

elisp

;; WRONG: operates on agent-shell buffer
(buffer-string)

;; RIGHT: explicitly target a buffer
(with-current-buffer "target-buffer"
  (buffer-string))

;; Or use a temp buffer for scratch work
(with-temp-buffer
  (insert-file-contents "/path/to/file")
  (buffer-string))

Periodic Background Tasks

Use run-at-time for scheduled operations:

elisp

;; Run once after 5 minutes
(run-at-time 300 nil #'my-function)

;; Run every 60 seconds
(run-at-time t 60 #'my-repeating-function)

;; Cancel a timer
(cancel-timer timer-object)

Coordinating Multiple Agents

If multiple agent-shell sessions exist:

elisp

;; List all agent buffers
(agent-shell-buffers)

;; Get project-specific buffers  
(agent-shell-project-buffers)

;; Queue request to a specific shell (safe even if busy)
(with-current-buffer "Claude Code Agent @ other-project"
  (agent-shell--enqueue-request :prompt "message"))

Inspecting Agent State

Access internal state for debugging or coordination:

elisp

(with-current-buffer (agent-shell--current-shell)
  ;; Available state keys
  (mapcar #'car agent-shell--state)
  ;; => (:agent-config :buffer :client :request-count :pending-requests ...)
  
  ;; Check request count
  (alist-get :request-count agent-shell--state)
  
  ;; Check if authenticated
  (alist-get :authenticated agent-shell--state))

Error Handling

Errors return with isError: true. To handle gracefully in Elisp:

elisp

(condition-case err
    (potentially-failing-operation)
  (error (format "Failed: %s" (error-message-string err))))

Event-Driven Agent Behavior (Hooks)

Emacs's hook system enables event-driven agent behavior:

elisp

;; OBSERVE user actions
(add-hook 'after-save-hook #'agent-on-file-saved)
(add-hook 'compilation-finish-functions #'agent-on-compile-done)
(add-hook 'flymake-diagnostic-functions #'agent-on-diagnostics)
(add-hook 'buffer-list-update-hook #'agent-on-focus-change)

;; INTERVENE at key moments  
(add-hook 'before-save-hook #'agent-maybe-format)
(add-hook 'find-file-hook #'agent-setup-buffer)

;; ADVICE any function non-invasively
(advice-add 'compile :before #'agent-log-compile-command)
(advice-add 'save-buffer :after #'agent-notify-save)

This enables proactive assistance without polling - the agent becomes an event-driven observer that reacts to user actions in real-time.

Key Insight: Dynamic Modification

Unlike static languages, Elisp lets agents modify behavior at runtime:

elisp

;; Add logging to ANY function, instantly, no rebuild
(advice-add 'some-function :before 
  (lambda (&rest args) (message "Called with: %S" args)))

;; Remove it just as easily
(advice-remove 'some-function #'my-advice-fn)

This is the core advantage over TypeScript/VS Code extensions - no compilation, no packaging, no restart. The agent can experiment and iterate in real-time.

Repository Structure

This is a literate Emacs configuration using Org-Babel. Assumes Emacs 30+ as baseline.

Configuration Hierarchy

All modules are loaded at startup via org-babel-load-file:

code

init.org (2,288 lines) - MAIN ENTRY POINT
├── emacs24.helm.org - Helm completion
├── emacs24.smartparens.org - Structural editing
├── emacs24.hacky.org - Misc utilities
├── emacs24.python.org - Python dev
├── emacs24.eshell.org - Shell with Git prompt
├── emacs24.ess.org - R/statistics
├── emacs24.elisp.org - Elisp dev
├── emacs24.js2.org - JavaScript
├── emacs24.sql.org - SQL (5 formatters, sqlfluff preferred)
├── emacs24.scheme.org - Scheme
├── emacs29.llm.org - LLM/AI integration
└── private.org - API keys (external, not in repo)

Note: "emacs24" naming is historical, not version-specific.

Key Sections in init.org

Lines	Purpose
1-150	Core setup: GC, keybindings, platform detection, straight.el
150-450	Org mode configuration
374-450	External module loading
450-2100	Language modes, UI, navigation
2100+	Themes, finalization, auto-tangle

Runtime Data (do not commit)

•bookmarks - Emacs bookmark locations
•keyfreq - Key frequency statistics
•prompts/ - LLM prompt templates
•snippets/ - Yasnippet templates
•elpa/, straight/repos/ - Package caches
•eln-cache/ - Native compilation cache

Package Management

Uses straight.el with use-package for declarative configuration. ~116 packages configured.

Known Issues (documented for cleanup)

•emacs24.eshell.org: Git status runs shell commands on every keystroke (performance)
•emacs24.hacky.org: date2unix defined twice; legacy keyboard macros
•emacs24.scheme.org: xscheme is ancient/unmaintained
•emacs24.js2.org: js2-mode superseded by js-ts-mode