ast-mcp-server

ast-mcp-server gives coding agents compact, type-aware access to TypeScript and JavaScript projects. It uses the real compiler project model through ts-morph, so declarations, references, rename locations, and diagnostics come from the AST instead of text-search guesses.

Reads are bounded and structured. Writes follow an explicit prepare → review → apply protocol with immutable hashes, workspace freshness checks, diagnostic guards, and idempotent receipts.

The problem

Coding agents often fall back to two generic operations: read files as plain text and write text patches. That works, but it has three predictable costs:

1. Too much context. The agent may load hundreds of lines when it only needs one signature or method body. That consumes model context and tokens without improving the answer.
2. Fragile edits. Text patches do not inherently understand declarations, scopes, overloads, or TypeScript diagnostics. A plausible-looking edit can target the wrong construct or introduce a new compiler error.
3. Weak cross-file reasoning. Text search can find matching words, but it cannot reliably distinguish two unrelated symbols with the same name. Project-wide references and renames need the compiler's understanding of the program.

What this tool does instead

This MCP server gives the agent structural code tools in addition to generic file reads and writes. Under the hood, ts-morph uses the TypeScript compiler project model, so the server can reason about declarations and references as code rather than undifferentiated text.

Need	Structural operation	Returned scope
Understand a file	ast_get_outline	Signatures without implementation bodies
Inspect one declaration	ast_get_symbol_source	Exact source for one function, method, class, or type
Find usages across the project	ast_find_references	Compiler-resolved reference locations
Rename a symbol everywhere	ast_rename_symbol	A reviewed project-wide rename plan
Change one implementation	ast_replace_symbol_body	A body-only plan that preserves the declaration

Reads can start with a compact outline and fetch exact source only when needed. Mutations are prepared in memory first, compared against baseline diagnostics, and returned as immutable, hash-bound plans. Nothing is written until the caller reviews and explicitly applies the plan.

Why this helps

• Less context: the agent retrieves the smallest structural unit that answers the question instead of loading the complete file by default.
• Safer changes: exact symbol selection, diagnostic deltas, workspace freshness checks, and prepare → review → apply reduce the failure modes of ad hoc text editing.
• Accurate project-wide operations: references and renames use compiler resolution rather than matching identifier text with grep.

AST-aware editing is not a proof that a change is semantically correct. The safety comes from combining structural selection with diagnostics, exact previews, reviewed hashes, freshness checks, and fail-closed apply semantics.

The included batch benchmark records a 50% reduction in model round-trips and a 95.21% reduction in serialized context for its search-to-source scenario. The format benchmark records an o200k_base estimate of 25.72% fewer model-facing tokens for a 100-row symbol search and 26.56% across the eligible collection corpus. These are reproducible scenario measurements, not universal token, billing, cache, or latency claims.

Requirements

• Node.js 20.19 or newer
• Corepack with Yarn 4.15.0 (pinned by packageManager)
• A target project with a tsconfig.json

Install

Install the published CLI globally while keeping dependency lifecycle scripts disabled:

npm install --global ast-mcp-server --ignore-scripts
ast-tool setup

--ignore-scripts prevents dependencies from running preinstall, install, or postinstall hooks. The package and its current runtime dependencies do not require those hooks.

Install from source

To build the current source instead:

git clone https://github.com/yailPeralta/ast-mcp-server.git
cd ast-mcp-server
corepack enable
yarn install --immutable
yarn build

The repository pins Yarn 4 and commits enableScripts: false in .yarnrc.yml. Dependency lifecycle scripts are therefore disabled during installation; switching from npm without this setting would merely change logos while preserving the risk.

The package exposes two executables when installed:

• ast-mcp-server: MCP stdio server.
• ast-tool: batch, skill-installation, and agent-setup CLI.

Guided agent setup

The installed package opens the interactive wizard with:

ast-tool setup

From a source checkout, use the Yarn script; it builds first and then opens the same wizard:

yarn setup

The wizard detects Claude Code and Hermes from PATH, shows their executable and version, selects every detected agent by default, and lets you deselect agents before confirmation. For each selected agent it:

1. preflights any existing ast MCP registration;
2. installs the bundled structural-code-editing skill;
3. registers this package's MCP server through the agent's official CLI;
4. reconnects and verifies the expected tools.

Existing matching registrations and skill files are unchanged. Conflicting MCP registrations fail before any write; remove or rename them explicitly instead of letting a setup script guess. Conflicting skill content also fails closed unless --force-skill is explicit.

For automation, make the target set and confirmation explicit:

ast-tool setup --agents all --yes
ast-tool setup --agents claude --yes

From a source checkout, replace ast-tool with yarn setup in those commands.

Install the agent skill

The package bundles a structural-code-editing skill that teaches an agent when to use the AST tools, how to minimize context, and how to review mutations safely. Install it for both Claude Code and Hermes with one command:

ast-tool install-skill all

Or install one target at a time:

ast-tool install-skill claude
ast-tool install-skill hermes

The default is user scope. It writes to Claude Code's personal skill directory and to the active HERMES_HOME:

Target	Destination
Claude Code	$CLAUDE_CONFIG_DIR/skills/structural-code-editing/SKILL.md, or ~/.claude/skills/... by default
Hermes	$HERMES_HOME/skills/software-development/structural-code-editing/SKILL.md, or ~/.hermes/... by default

To commit the skill into one project for Claude Code, use project scope:

ast-tool install-skill claude --scope project --project-root /absolute/project

This writes .claude/skills/structural-code-editing/SKILL.md below that project. Project scope is intentionally rejected for Hermes because Hermes skills belong to a profile, not a source repository.

Installation is idempotent. Existing identical content is left untouched; different content fails closed unless --force is explicit. From an unlinked source checkout, replace ast-tool with yarn node /absolute/path/to/ast-mcp-server/dist/cli.js.

Claude Code detects changes in an existing skill directory live; restart it if the top-level skills directory did not exist when the session started. In Hermes, run /reload-skills or start a new session, then verify with hermes skills list.

install-skill only installs the skill; it does not configure the MCP transport. Use the guided setup command to do both, or complete the client-specific MCP setup below—the instructions are useful, but they have not yet learned to open a stdio socket through positive thinking.

Use with Claude Code

Claude Code supports local stdio MCP servers. After building this repository, register the server with an absolute entrypoint:

AST_MCP_DIR="$(pwd)"
claude mcp add --scope user --transport stdio ast -- \
  node "$AST_MCP_DIR/dist/index.js"
claude mcp get ast

claude mcp get ast should report Status: ✔ Connected. The -- separator is required: everything after it is the server command, not a Claude Code option.

The example uses --scope user, which makes the server available in all your projects. Use --scope local instead to register it only for the project from which you run the command. Avoid committing a project-scoped .mcp.json that contains another developer's absolute checkout path.

Start Claude Code inside any TypeScript project with a tsconfig.json:

cd /absolute/path/to/your-typescript-project
claude

Then ask Claude to use the ast tools. For example:

Use the ast MCP server to inspect this project.
First search for UserService, then fetch only the exact source of its create method.

For a reviewed rename:

Use ast_rename_symbol to prepare renaming UserService.create to createUser.
Do not apply it yet. Show me the affected files, diagnostic delta, plan hash,
and the complete operation preview.

After reviewing the preview:

Apply that operation with ast_apply_operation using the exact operation_id and
plan_hash returned by the prepare step.

Project-scoped read and prepare tools require project_root. Claude should pass the current project directory or its explicit tsconfig.json path. Preview and apply calls instead use the prepared operation coordinates; the MCP server itself contains no repository-specific paths.

MCP or batch CLI?

Workflow	Recommended interface
Interactive exploration or one reviewed mutation	Claude Code MCP tools
A known multi-step read pipeline	ast-tool run pipeline.json through Bash
Prepare now and apply in a later process	ast-tool run, then ast-tool apply

Use /mcp inside Claude Code to inspect server status and tools. Outside the session, use claude mcp list, claude mcp get ast, or claude mcp remove ast -s user.

Other MCP clients

Hermes Agent:

hermes mcp add ast --command node --args /absolute/path/to/ast-mcp-server/dist/index.js
hermes mcp test ast

Project-scoped tools accept project_root, either the project directory or an explicit tsconfig.json path. The server contains no repository-specific paths.

MCP tools

Tool	Purpose	Mutates files
ast_list_files	Paginated, project-relative source file inventory	No
ast_get_outline	Body-free declaration signatures; detailed symbol metadata is opt-in	No
ast_get_symbol_source	Exact source for one declaration	No
ast_search_symbols	Paginated structural symbol discovery	No
ast_find_references	Compiler-resolved references with bounded context	No
ast_get_diagnostics	Project- or file-scoped TypeScript diagnostics	No
ast_rename_symbol	Prepare a project-wide rename	No
ast_replace_symbol_body	Prepare a body-only replacement while preserving the signature	No
ast_get_operation_preview	Retrieve the complete retained diff for a prepared plan	No
ast_apply_operation	Apply one reviewed, hash-bound plan	Yes

Read results use project-relative paths, deterministic ordering, structured MCP output, and pagination where result sets can grow with the project.

Optional TOON results

ast_search_symbols, ast_find_references, and ast_get_diagnostics accept output_format: "toon" for collection-heavy results consumed directly by a model. JSON remains the default and preserves the canonical structured object.

MCP TOON is returned once as structured content shaped like { "format": "toon", "data": "..." }; data is the lossless TOON document. The complete JSON result is not duplicated. These three tools validate their canonical Zod result and verify an encode/decode deep-equality round trip before presentation, but do not advertise a single MCP outputSchema because their successful structured content has two representations.

Do not request TOON for source, outlines, file lists, previews, or mutation results. Checked negative controls show that the MCP envelope makes those shapes larger. TOON is an explicit shape-specific optimization, not a new dialect for every object in sight.

Batch CLI

ast-tool lets Claude Code and other Bash-capable clients collapse a known structural pipeline into one shell call:

ast-tool validate pipeline.json
ast-tool run pipeline.json
ast-tool run pipeline.json --output-format toon
cat pipeline.json | ast-tool run -

Example search-to-source pipeline:

{
  "version": 1,
  "project_root": "/absolute/project",
  "steps": [
    {
      "id": "search",
      "tool": "ast_search_symbols",
      "input": { "query": "UserService", "limit": 20 }
    },
    {
      "id": "source",
      "tool": "ast_get_symbol_source",
      "input": {
        "file_path": { "$ref": "#/steps/search/symbols/0/file" },
        "symbol_path": { "$ref": "#/steps/search/symbols/0/symbol_path" }
      }
    }
  ],
  "emit": { "$ref": "#/steps/source" }
}

A $ref is an RFC 6901 JSON Pointer rooted at prior step results. References cannot point forward. If emit is omitted, only the final step result is returned; intermediate results remain inside the process.

Bounded foreach

{
  "version": 1,
  "project_root": "/absolute/project",
  "limits": { "concurrency": 4 },
  "steps": [
    { "id": "files", "tool": "ast_list_files", "input": { "limit": 20 } },
    {
      "id": "outlines",
      "tool": "ast_get_outline",
      "foreach": { "$ref": "#/steps/files/files" },
      "input": { "file_path": { "$item": "" } }
    }
  ]
}

$item accepts an empty pointer for the complete item or /field for one field. Foreach is read-only, order-preserving, fail-fast, and concurrency-bounded.

Batch limits

• Input document: 1 MiB.
• Steps: 50.
• Total tool invocations: 500.
• Foreach items per step: 200.
• Read concurrency: default 4, maximum 16.
• Each retained step result and final serialized output: 10 MiB.
• Total retained intermediate context: 50 MiB.
• One project root per pipeline.
• No branches, eval, embedded JavaScript, while loops, or arbitrary transformations.

Success is one compact JSON value on stdout by default. ast-tool run --output-format toon writes one plain TOON document for a read-only batch; internal steps remain structured JSON, and prepare batches reject TOON before execution. Encoding and output-limit failures write no partial stdout and use stable ENCODING_ERROR or OUTPUT_LIMIT codes. Errors are always structured JSON on stderr. Exit code 0 is success, 1 is execution/apply failure, and 2 is usage or schema failure.

Reviewed mutations

MCP process

Rename and body replacement never write directly:

1. Call ast_rename_symbol or ast_replace_symbol_body.
2. Review the diagnostic delta, affected files, blocked, and plan_hash.
3. Fetch complete diffs with ast_get_operation_preview when needed.
4. Call ast_apply_operation with both operation_id and plan_hash.

MCP plans live in a bounded in-memory store and do not survive a server restart.

CLI process boundary

A batch may contain at most one prepare operation. It must be the final step and cannot use foreach. ast_apply_operation and arbitrary preview calls are forbidden inside batch documents.

A CLI prepare writes an exact private plan and returns top-level operation_id, plan_hash, and plan_file even when emit omits them:

ast-tool run prepare-rename.json
ast-tool apply /path/from/plan_file.astplan --plan-hash <reviewed-sha256>

The default plan directory is:

${XDG_STATE_HOME:-~/.local/state}/ast-tool/plans

Set AST_TOOL_STATE_DIR to isolate it. Directories are mode 0700; plans are mode 0600, atomically replaced, size-bounded, versioned, and expire with the prepared operation. Plan files contain exact proposed source bytes and must be treated as private code.

Apply loads the exact retained postimages, requires the separately supplied reviewed hash, validates serialized byte hashes and contained paths, rechecks the complete source/config workspace, stages writes, verifies postimages, and persists an applied receipt inside the same cooperative workspace lock. A later CLI invocation can replay that receipt idempotently, including after the preparation TTL.

Guarantee boundary

The server does not claim a filesystem-wide transaction:

• Replacement is atomic per file where the local filesystem provides atomic rename.
• A multi-file apply has a short interval in which some replacements may already be visible.
• Rollback is best effort and refuses to overwrite a file changed by another writer after replacement.
• MCP and CLI apply share a fail-closed filesystem lock keyed by canonical tsconfig.json when they use the same state directory. It does not coordinate editors, NFS writers, or hostile external processes.
• Receipt persistence runs before that lock is released. If receipt storage fails after source replacement, apply exits non-zero and reports that verified postimages may be present; retry recovers the receipt only when the complete workspace exactly matches the reviewed post-workspace fingerprint.
• A hard process crash can leave a stale lock. Remove it only after inspecting its metadata and proving no apply is running; exact complete postimages can then recover the receipt, while partial or divergent state remains a conflict.
• Source encoding support is UTF-8, with or without BOM. Unsupported encodings are rejected.

Development gates

yarn format:check
yarn lint
yarn typecheck
yarn test
yarn build
yarn test:mcp
yarn test:cli
yarn test:package
yarn npm audit --all --recursive
yarn pack --dry-run

test:mcp exercises the built stdio server. test:cli runs a read pipeline and a prepare/apply/replay workflow across separate Node processes.

Benchmarks

yarn benchmark /absolute/project --sample 20 --output benchmark/results/project.json
yarn benchmark:corpus benchmark/task-corpus.json --output benchmark/results/self-corpus.json
yarn benchmark:batch --iterations 5 --output benchmark/results/self-batch.json
yarn benchmark:formats

The batch benchmark compares two separate client calls with one batch invocation in fresh Node processes, recording model round-trips, actual tool invocations, wall time, maximum RSS, and serialized character counts. Character counts are not model-specific token estimates.

The format benchmark runs real tools against this repository plus deterministic reference/diagnostic fixtures. It checks JSON→TOON→value equality, UTF-8 bytes, gpt-tokenizer o200k_base estimates, encode/decode latency, the actual MCP envelope, tool metadata, and negative controls. Its checked result is benchmark/results/self-formats.json; local tokenizer estimates do not establish provider-side billing or cache savings. See benchmark/README.md for methodology and limitations.

Scope

• TypeScript and JavaScript projects understood by the TypeScript compiler.
• Structural rename and callable-body replacement.
• Declarative DAG-like pipelines with prior-result references and bounded foreach.
• No arbitrary signature migration, file creation/deletion plans, cross-language refactors, or general-purpose scripting language.