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Tessera

MCP server

Terminal window
tessera mcp                          # stdio
tessera mcp --http 127.0.0.1:8765    # Streamable HTTP on POST /mcp, health on GET /health

Both transports start answering at once. The first index builds in the background; until it exists, tool calls return isError: true with “Index is building … retry in a few seconds”. After that, a file watcher re-indexes changed files within about 200 ms.

Protocol

Section titled “Protocol”

Tessera is a dual-era server:

  • Modern clients (2026-07-28). Requests carry _meta["io.modelcontextprotocol/protocolVersion"] and are served statelessly. server/discover lists the supported versions. An unsupported version gets error -32022 with the supported list.
  • Legacy clients (2025-11-25, 2025-06-18, 2025-03-26, 2024-11-05). They use the initialize handshake. Tessera echoes the requested version when it supports it, and otherwise answers 2025-11-25.

Errors follow JSON-RPC:

Code Meaning
-32700 parse error
-32600 invalid request; batches are not supported
-32601 unknown method
-32602 unknown tool or invalid arguments

A tool that runs but fails returns a normal result with isError: true, so the model can read the message and recover. Notifications never get a reply.

HTTP specifics:

  • Origin must be absent or localhost / 127.0.0.1 / [::1]; any other origin gets 403.
  • Notifications get 202.
  • GET and DELETE on /mcp get 405.
  • Modern requests must send MCP-Protocol-Version, Mcp-Method, and, for tools/call, Mcp-Name, all matching the body. A missing or mismatched header gets 400 with code -32020.

Tools

Section titled “Tools”

Every tool is read-only (readOnlyHint: true) and rejects unknown arguments. Responses are compact text by default; pass "format": "json" for machine-readable output. List tools are paginated: limit (default 25) and cursor. When there is more, the text ends with cursor o:N.

Tool Use it for Arguments
find Where a symbol is defined: kind, file:line, signature, optional body. With path, outline a file or directory plus its imports. symbol or path, include_body
search Fuzzy or glob name search with filters. unused: true lists functions nothing calls. pattern, kinds, languages, path, exported, unused
refs Every reference to a symbol with its confidence, or the files importing a file. target, tests_only, min_confidence
blast Callers ranked by impact, plus test files to run, for a symbol or a git diff. symbol or diff_from (+diff_to), depth
verify Does a symbol exist; does every call in a snippet resolve; did a change break a call site, an import, or a contract? symbol, snippet + language, or changed / staged / diff_from
context Body, callees, callers, tests, and siblings in one budgeted bundle. symbol, budget_tokens
path Shortest call chain from one symbol to another, across services through routes and gRPC calls. from, to, depth
map Contracts between services: routes, gRPC methods, topics, tables, with both sides; or what a service serves and calls. kind (route, rpc, topic, table, service), query
status Index root, freshness, counts, and setup hints. none
import_cycles Circular dependency cycles between imported modules. path, limit
affected The test files a change reaches, each with the call path (across contracts) that selects it; changed and failed tests always run, lockfile, build, config and fixture changes run everything. rev or staged, failed, explain, package, depth

Symbols can be written name, Class.method, or A::B. References and callers are resolved through scope, imports, and declared types, and each carries a confidence: unmarked lines are exact, “likely” lines matched a unique name, “possible” lines are ambiguous or name-level. See accuracy.

Checking conformance yourself

Section titled “Checking conformance yourself”

The repository’s tests/mcp_conformance.rs drives the binary through full stdio and HTTP sessions. To poke the server interactively:

Terminal window
npx @modelcontextprotocol/inspector tessera mcp
npx @modelcontextprotocol/inspector --cli tessera mcp --method tools/list