Type Atlas

[Type Atlas — code intelligence for TypeScript]

Type Atlas is what I have all my code agents use across all of my TypeScript projects for all code navigation needs. Most of those projects are monorepos. Some are large and complex enough that understanding how a change fits into the rest of the system is a real part of the work. This tool was designed as a complete replacement for an agent's default code navigation methodology.

I've been iterating on this tool for months based on how my own coding agents actually work in my projects. Most of what Type Atlas does, exists because I kept seeing the same problems:

• Agents reason from an incomplete view of the system. They may understand the file they found while missing the surrounding code that determines how it is supposed to be used.
• Agents rebuild things that already exist. The capability is already in the repo, but the agent never finds it because it does not know what it is called.
• Agents often read either too little code (Claude) or far too much (Codex). Too little leaves them making decisions without enough surrounding context. Too much fills the context window with entire files and implementation detail that never mattered to the task.
• Agents repeatedly stop to run type checks just to find errors an IDE would already be showing. That adds latency throughout implementation and pushes feedback later than it needs to arrive.
• Agents are fundamentally operating blind. String-search navigation never gives them a strong internal atlas of the codebase. They are forced to make implementation decisions from fragments of files and matches they happened to retrieve, with important structure and compiler-known information still missing.

As projects grow, these problems compound and start showing up directly in the quality of the code agents write.

Semantic code navigation

Coding agents navigate code through file reads and string searches by default. That gives them source text and leaves the model to reconstruct relationships that the TypeScript language service already knows.

Type Atlas gives the agent direct access to that semantic information:

• A symbol resolves to its actual definition and references rather than every occurrence of the same text.
• Callers and implementations are identified by their relationship to the symbol.
• Inferred types come from the language service instead of being reconstructed from nearby source.
• Results keep their source range and owning TypeScript project attached.

A text match is still useful when text is what the agent is looking for. It is a weak substitute for semantic navigation when the question is about the program itself.

Find what already exists

Large codebases contain useful code that an agent has no reason to know by name. String search works best after the agent already knows enough vocabulary to formulate the search.

Type Atlas gives it other ways in:

• The agent can describe behavior in natural language and find code based on what it does.
• A discovered result leads back to a real symbol and an exact source range.
• From that symbol the agent can follow actual relationships through the codebase instead of guessing another identifier to search for.

This is especially useful in large monorepos. Existing helpers and established implementations are easier to discover before the agent decides it needs to create another one.

Diagnostics during normal work

A developer in an IDE sees compiler feedback while working. Coding agents usually get that feedback by stopping implementation to run a type-check command and then waiting for the result.

Type Atlas moves much of that feedback into work the agent was already doing:

• Relevant diagnostics can arrive with normal code-intelligence responses.
• Errors show up while the affected code is still part of the agent's current working context.
• A bad type assumption can be caught before it turns into several dependent edits.

I still use full type checks for verification. They do not need to be the main way an agent learns about errors while it works.

Responses are built for model context

Making a response smaller is useful only when the information removed was unnecessary. Models also use the organization of what remains.

Type Atlas treats structure as part of the information:

• Labels make the role of a result explicit.
• Grouping keeps related facts together.
• File boundaries stop unrelated source from blending together.
• Source locations stay attached to the thing they describe.
• Trees keep their hierarchy instead of becoming a flat sequence.
• Diagnostics retain the source needed to understand them.

The same principle determines what gets omitted. Function bodies can stay folded when the signature is enough. Repeated serialization and unrelated source do not need to occupy the context window simply because they were available.

The goal is useful information density. Fewer tokens matter, but removing the structure that helps the model understand those tokens would defeat the point.

Information for the next decision

Every tool call is part of the agent's reasoning process. A good response should answer the current question while leaving the agent in a better position to decide what to inspect next.

Type Atlas keeps useful follow-up information close to the result that exposed it:

• A symbol can arrive with the relationships needed to understand how it participates in the codebase.
• Repository structure can carry line counts and working-tree state alongside the files themselves.
• Search results include concrete source ranges that can be followed directly.
• Project context remains attached as the agent moves from one result to another.

This gives the agent better evidence at each branch in its investigation. It can follow relationships that actually exist in the program instead of treating every textual match as an equally meaningful lead.

The benefit is higher-quality navigation. Each step preserves more of the information needed to choose the next one.

Project and scope awareness

TypeScript questions depend on project context. That becomes especially important in a monorepo, where an answer can be correct inside one project while still being incomplete for the repository as a whole.

Type Atlas keeps those boundaries visible:

• Files are resolved through the TypeScript project that owns them.
• Results state the project scope they came from when that scope matters.
• Counts make the size of a result explicit.
• Limits are stated when an answer covers less than the whole repository.
• Source locations can be passed directly into later calls.

The agent gets enough information to understand what an answer actually covers before relying on it.

Built from daily agent use

I use Type Atlas with my own coding agents every day across all of my TypeScript projects. The current behavior came from repeated use.

A lot of the design can be traced directly to recurring agent behavior:

• Reads fold because agents were spending context on bodies they did not need.
• Diagnostics travel with normal responses because repeated type-check commands were wasting implementation time.
• Semantic relationships are grouped because agents kept reconstructing the same information through separate calls.
• Natural-language code search exists because useful code often has a name the agent could never infer from the task.

That is still how I work on Type Atlas. When I keep seeing agents waste time on the same navigation problem or repeatedly miss the same kind of information, I change the tool.

The examples below are captured from the running server against a fixture monorepo and regression-checked with the implementation.

Install

codex mcp add type-atlas -- npx --yes @type-atlas/mcp@latest
claude mcp add --scope user type-atlas -- npx --yes @type-atlas/mcp@latest
code --add-mcp '{"name":"type-atlas","command":"npx","args":["--yes","@type-atlas/mcp@latest"]}'

Any other client takes the standard shape:

{  "mcpServers": {    "type-atlas": {      "command": "npx",      "args": ["--yes", "@type-atlas/mcp@latest"]    }  }}

A client that starts servers without your shell PATH will not find npx by name; give the absolute path from which npx when that happens. On Windows, a client that cannot launch the npx.cmd shim needs "command": "cmd" with "args": ["/c", "npx", "--yes", "@type-atlas/mcp@latest"].

Clients read MCP config at startup, so restart after. @latest resolves on every process start; pin a version if you do not want tool behavior moving under you.

search_code, related_code, investigate_code, and search_dependency_code run a semantic index through uvx and need uv. Without it those four report that uv is missing, explore_symbol drops its related-code section, and the rest is unaffected.

Recommended

Installing the server does not change what an agent reaches for. Some agents, Claude among them, will assemble whatever their shell allows, chained together, and produce a fresh justification each time, so naming a few commands to avoid does not hold. The instruction has to rule out the whole category and name the exceptions. Add this to AGENTS.md or CLAUDE.md:

Type Atlas MCP is the required tool for reading and navigating code in TypeScript and JavaScript. This is not a preference. No shell command is an acceptable substitute, whatever it is composed of, and neither is a plain file read. The only valid fallbacks are a server that is down, a call that errored, or a file that is neither TS nor JS.

--require-intent

This opt-in flag requires one decision sentence for broad exploration tools such as repository search and workspace symbols. Targeted reads and semantic lookups stay unaffected, and intent is never echoed into tool responses.

Tool call results

Paths are workspace-relative, coordinates are one-based, so a location in one answer is valid input to the next call. Editing tools return patches; nothing is written for you.

Everything below is captured from the running server against fixtures/ledger by the scenario suite, which replays the same calls and fails on drift. Nothing here is hand-written, and changing what a tool answers changes this file in the same commit. The source is README.mdoc. Every tool has a page with more cases in docs/tools.

list_files

Structure, line counts, and git status in one tree, using the badge letters editors already use. Deleted files get a row even though they exist only in git's answer. Folded directories say what they hold rather than disappearing.

Agent's Input

tool: List filesworkspace: fixtures/ledger# working tree arranged: currency.ts edited · rounding.ts created · index.ts deleteddirectory: packages/moneydepth: 2
# answered in 57ms

Response

packages/money/├  src/ · 3 changed│  ├  currency.ts · 21 loc · M +2│  ├  index.ts · D -12│  ├  money.ts · 58 loc│  ├  rounding-mode.ts · 15 loc│  └  rounding.ts · 11 loc · U├  tests/│  ├  money.test.ts · 15 loc│  └  rounding-parity.ts · 15 loc├  package.json · 19 loc└  tsconfig.json · 20 loc

inspect_symbol

Hover, definitions, type definitions, implementations, callers, calls, and references in one call. References are the residual after callers and definitions are accounted for, so a use is listed once. Against calling those tools separately it is 4x fewer characters and 7x fewer round trips.

Agent's Input

tool: Inspect symbolworkspace: fixtures/ledgerfile: packages/accounts/src/journal.tssymbol: Journal
# answered in 49ms

Response

Journal [class] · packages/accounts/src/journal.ts:24:14-24:21 · range 24:1-73:2 · packages/accounts/tsconfig.json
```typescriptclass Journal<TMeta = undefined>```
An append-only journal of balanced entries. `TMeta` carries whatever aconsumer attaches to each entry — an import batch id, an approval trail —without the journal knowing its shape.
## Callers (4)
packages/accounts/tests/journal.test.ts├  test("posts a balanced transfer through the overload") callback [function] 5:56-14:2 · calls 6:23-6:30└  test("refuses an unbalanced entry") callback [function] 16:37-29:2 · calls 17:23-17:30packages/reports/src/balance.ts└  balancesAsOf [variable] 23:14-23:26 · range 23:14-51:2 · calls 24:12-24:19packages/importers/src/csv.ts└  importStatement [variable] 28:14-28:29 · range 28:14-47:2 · calls 29:12-29:19
## Mentions that are not calls (4 of 9 references · 5 relevant projects searched)
packages/accounts/tests/journal.test.ts:3:25-3:32:  import { credit, debit, Journal, UnbalancedEntryError } from "../src/index.ts";packages/accounts/src/index.ts:11:22-11:29:  export { type Entry, Journal, UnbalancedEntryError } from "./journal.ts";packages/reports/src/balance.ts:4:8-4:15:  type Journal,packages/importers/src/csv.ts:1:10-1:17:  import { Journal, type Entry, credit, debit, type AccountPath } from "@ledger/accounts";
references lists all 9, with paging.

read_file

The argument is an array, so several files arrive in one call. Bodies fold to signatures by default and the header says how many lines that saved; fold: false returns them.

Agent's Input

tool: Read filesworkspace: fixtures/ledgerfile: ["packages/accounts/src/posting.ts","packages/money/src/rounding-mode.ts"]
# answered in 7ms

Response

2 files · 42 lines · 6 folded to signatures, pass fold: false for the bodies
=== packages/accounts/src/posting.ts · 32 lines ===
 1 | import { type Money, negate } from "@ledger/money"; 2 | import type { AccountPath } from "./account.ts"; 3 | 4 | /** 5 |  * One side of a journal entry. The discriminant is the bookkeeping side, so 6 |  * every consumer's switch is checked for exhaustiveness by the compiler. 7 |  */ 8 | export type Posting = 9 |   | { readonly side: "debit"; readonly account: AccountPath; readonly amount: Money }10 |   | { readonly side: "credit"; readonly account: AccountPath; readonly amount: Money };11 |12 | export const debit = (account: AccountPath, amount: Money): Posting => ({13 |   side: "debit",14 |   account,15 |   amount,16 | });17 |18 | export const credit = (account: AccountPath, amount: Money): Posting => ({19 |   side: "credit",20 |   account,21 |   amount,22 | });23 |24 | /** A posting's effect on a debit-normal running balance. */25 | export const signedAmount = (posting: Posting): Money => {   |   ... 26-31 folded32 | };
=== packages/money/src/rounding-mode.ts · 15 lines ===
 1 | /** How sub-minor precision resolves when a statement and the books disagree. */ 2 | export enum RoundingMode { 3 |   HalfUp = "half-up", 4 |   HalfEven = "half-even", 5 |   Truncate = "truncate", 6 | } 7 | 8 | /** Per-institution conventions, as observed in their exports. */ 9 | const bankRounding: Readonly<Record<string, RoundingMode>> = {10 |   "first-national": RoundingMode.HalfEven,11 |   "harbor-credit": RoundingMode.HalfUp,12 | };13 |14 | export const roundingModeOf = (bank: string): RoundingMode =>15 |   bankRounding[bank] ?? RoundingMode.HalfEven;

occurrences

Starts from an exact identifier when the agent knows its name but not its file. Identifier syntax nodes activate the relevant projects; TypeScript resolves canonical declarations and semantic references. Same-name symbols stay separate, overloads collapse, and unresolved identifiers are called out rather than mixed into the reference list.

Agent's Input

tool: Occurrencesworkspace: fixtures/ledgerquery: moneylimit: 12
# answered in 244ms

Response

Identifiers: "money"
Scope: workspace · 33 project files
Showing 1–12 of 22 references · next offset: 12
=== money [function] · packages/money/src/money.ts:27:14 · 12/22 references shown ===
24 exact-name locations across 8 files · 1 without a workspace declaration
Locations without a workspace declaration (1):
packages/reconcile/src/matching.ts:24:37 — inside emptyRemainder · export const emptyRemainder = () => money(0, "USD");
packages/importers/src/csv.ts├  2:37  — at module level · import { type Currency, isCurrency, money, zero, format } from "@ledger/money";└  34:20 — inside amount · const amount = money(Math.abs(row.amountMinor), row.currency);packages/money/src/index.ts└  8:3 — at module level · money,packages/money/src/money.ts├  30:52 — inside zero · export const zero = (currency: Currency): Money => money(0n, currency);├  42:10 — inside add · return money(left.minorUnits + right.minorUnits, left.currency);└  45:48 — inside negate · export const negate = (value: Money): Money => money(-value.minorUnits, value.currency);packages/reconcile/src/drift.ts├  5:18  — at module level · import { format, money, type Money } from "@ledger/money";└  21:17 — inside drift · return format(money(journalTotal - statementTotal(statement), "usd"));packages/accounts/tests/journal.test.ts├  1:10  — at module level · import { money } from "@ledger/money";├  9:71  — inside test("posts a balanced transfer through the overload") callback · { from: "assets:bank:checking", to: "expenses:furniture", amount: money(24900, "USD") },├  23:34 — inside expect() callback · debit("expenses:travel", money(5000, "USD")),└  24:40 — inside expect() callback · credit("assets:bank:checking", money(500, "USD")),

search_code

Finds code by what it does, for when you cannot guess what it is called. Hits come back in rank order, each carrying the file range it came from, so the next call has somewhere to go. Live answers also carry a relevance percentage per hit; it is left out below because the embedding scores behind it differ between machines and these cases are compared byte for byte.

Agent's Input

tool: Search codeworkspace: fixtures/ledgerquery: walking an account up through each of its ancestor accountssnippetLines: 6
# answered in 20ms

Response

Search: walking an account up through each of its ancestor accounts
5 matches · no identifier to anchor on, so these are ranked by meaning alone
=== 1 · packages/accounts/src/account.ts:21-35 ===
Structure: parentPathSymbol: parentPath [variable] · selection 21:14-21:24 · range 21:14-24:2
21 | export const parentPath = (path: AccountPath): AccountPath | undefined => {22 |   const at = path.lastIndexOf(":");23 |   return at === -1 ? undefined : path.slice(0, at);24 | };25 |26 | /** Every ancestor from root to the account itself: `a`, `a:b`, `a:b:c`. */
=== 2 · packages/reports/src/balance.ts:1-23 ===
Structure: BalanceLineSymbol: BalanceLine [interface] · selection 11:18-11:29 · range 11:1-16:2
1 | import {2 |   type AccountPath,3 |   type Entry,4 |   type Journal,5 |   lineage,6 |   signedAmount,
=== 3 · packages/accounts/src/journal.ts:59-73 ===
Structure: Journal > historySymbol: history [method] · selection 60:3-60:10 · range 60:3-64:4
59 |   /** Entries touching an account, oldest first. */60 |   history(account: AccountPath): readonly Entry<TMeta>[] {61 |     return this.entries.filter((entry) =>62 |       entry.postings.some((posting) => posting.account === account),63 |     );64 |   }
=== 4 · packages/reports/src/statement.ts:1-11 ===
Structure: statementLineSymbol: statementLine [variable] · selection 8:14-8:27 · range 8:14-11:2
1 | import { type Account, normalBalance } from "@ledger/accounts";2 | import { format, type Money, negate } from "@ledger/money";3 |4 | /**5 |  * One rendered statement line. The sign follows the account's normal side:6 |  * a liability holding a credit balance reads as positive on its statement.
=== 5 · packages/accounts/src/posting.ts:1-24 ===
Structure: creditSymbol: credit [variable] · selection 18:14-18:20 · range 18:14-22:3
1 | import { type Money, negate } from "@ledger/money";2 | import type { AccountPath } from "./account.ts";3 |4 | /**5 |  * One side of a journal entry. The discriminant is the bookkeeping side, so6 |  * every consumer's switch is checked for exhaustiveness by the compiler.

diagnostics

The compiler's own whole-program check, per project, not a per-file pass. An edit in one file usually breaks a different one, and this is the call that finds that file.

Agent's Input

tool: Diagnosticsworkspace: fixtures/ledgerproject: packages/reconcilescope: project
# answered in 23ms

Response

Whole project · 6 problems in 2 files · 11 files checked · packages/reconcile/tsconfig.json
=== packages/reconcile/src/drift.ts ===
error typescript(2365) 16:33-16:52 — inside lines.reduce() callback  Operator '+' cannot be applied to types 'number' and 'Money'.   14 | /** Statement total, computed by someone who forgot Money is not a number.…   15 | export const statementTotal = (lines: readonly StatementLine[]): number =>   16 |   lines.reduce((total, line) => total + line.amount, 0);      |                                 ^^^^^^^^^^^^^^^^^^^   17 |   18 | /** Drift between the journal's view and the bank's view of one day. */
error typescript(2365) 20:77-20:91 — inside reduce() callback  Operator '+' cannot be applied to types 'import("packages/money/src/money").Money' and 'import("packages/money/src/money").Money'.   18 | /** Drift between the journal's view and the bank's view of one day. */   19 | export const drift = (postings: readonly Posting[], statement: readonly St…   20 |   const journalTotal = postings.map(signedAmount).reduce((total, amount) =…      |                                                                             ^^^^^^^^^^^^^^   21 |   return format(money(journalTotal - statementTotal(statement), "usd"));   22 | };
error typescript(2362) 21:23-21:35 — inside drift  The left-hand side of an arithmetic operation must be of type 'any', 'number', 'bigint' or an enum type.   19 | export const drift = (postings: readonly Posting[], statement: readonly St…   20 |   const journalTotal = postings.map(signedAmount).reduce((total, amount) =…   21 |   return format(money(journalTotal - statementTotal(statement), "usd"));      |                       ^^^^^^^^^^^^   22 | };   23 |
error typescript(2345) 21:65-21:70 — inside drift  Argument of type '"usd"' is not assignable to parameter of type 'Currency'.   19 | export const drift = (postings: readonly Posting[], statement: readonly St…   20 |   const journalTotal = postings.map(signedAmount).reduce((total, amount) =…   21 |   return format(money(journalTotal - statementTotal(statement), "usd"));      |                                                                 ^^^^^   22 | };   23 |
=== packages/reconcile/src/matching.ts ===
error typescript(2304) 14:20-14:32 — inside amount  Cannot find name 'signedAmount'.   12 |   const matched = new Map<Posting, StatementLine>();   13 |   for (const posting of postings) {   14 |     const amount = signedAmount(posting);      |                    ^^^^^^^^^^^^   15 |     const line = lines.find(   16 |       (candidate) => candidate.amount.minorUnits === amount.minorUnits,
error typescript(2304) 24:37-24:42 — inside emptyRemainder  Cannot find name 'money'.   22 |   23 | /** The zero of a matching pass, for currencies the statement never names.…   24 | export const emptyRemainder = () => money(0, "USD");      |                                     ^^^^^   25 |

workspace_symbols

Find a declaration by name across every project the session has loaded, when you know roughly what it is called and nothing about where it lives.

Agent's Input

tool: Workspace symbolsworkspace: fixtures/ledgerfile: packages/importers/src/statement-parser.tsquery: Parser
# answered in 100ms

Response

3 symbols matching Parser · 8 projects loaded · packages/importers/tsconfig.json
CsvStatementParser [class] · packages/importers/src/statement-parser.ts:25:1-35:2FixedWidthStatementParser [class] · packages/importers/src/statement-parser.ts:41:1-64:2StatementParser [class] · packages/importers/src/statement-parser.ts:7:1-23:2

file_references

Who imports this module. The module-level question, answered without picking a symbol inside it first.

Agent's Input

tool: File referencesworkspace: fixtures/ledgerfile: packages/money/src/money.ts
# answered in 134ms

Response

packages/money/src/money.ts · referenced from 90 places · 6 relevant projects searched · packages/money/tsconfig.json
1-20 of 90 places · pass offset: 20 for the rest
packages/accounts/src/journal.ts├  1:10  — at module level└  53:15 — inside postpackages/money/src/index.ts├  3:3 — at module level├  4:3 — at module level└  5:3 — at module levelpackages/money/tests/money.test.ts├  2:10  — at module level├  2:15  — at module level├  2:38  — at module level├  5:10  — inside test("adds amounts of one currency exactly") callback├  9:16  — inside expect() callback├  9:67  — inside test("refuses to combine currencies") callback├  13:10 — inside test("formats major and minor units per currency") callback└  14:10 — inside test("formats major and minor units per currency") callbackpackages/reports/src/balance.ts├  8:10  — at module level├  34:9  — inside balancesAsOf└  41:28 — inside balancesAsOfpackages/reports/src/statement.ts├  2:10  — at module level└  10:40 — inside statementLinepackages/rules/src/builtin.ts├  2:10  — at module level└  28:58 — inside closedPeriodsBalance

Packages

Development

vp installvp run checkvp run check:distribution

CONTRIBUTING.md has the change and release process.