FastMCP Development Guidelines

Audience: LLM-driven engineering agents and human developers

FastMCP is a comprehensive Python framework (Python ≥3.10) for building Model Context Protocol (MCP) servers and clients.

Required Development Workflow

CRITICAL: Always run these commands in sequence before committing.

uv sync                              # Install dependencies
uv run pytest -n auto                # Run full test suite

In addition, you must pass static checks. This is generally done as a pre-commit hook with prek but you can run it manually with:

uv run prek run --all-files          # Ruff + Prettier + ty + gitleaks

The gitleaks hook needs the gitleaks binary on PATH (brew install gitleaks, or see gitleaks releases) — it isn't a uv-managed dependency, so uv sync alone won't provide it.

Tests must pass and lint/typing must be clean before committing.

Repository Structure

Skills

Repository skills live in .agents/skills/, with symlinks in .claude/skills/ for Claude Code. The development guide says which work is automated and which needs a person. Load the skill that matches the job:

Core MCP Objects

When modifying MCP functionality, changes typically need to be applied across all object types:

• Tools (fastmcp_slim/fastmcp/tools/)
• Resources (fastmcp_slim/fastmcp/resources/)
• Resource Templates (fastmcp_slim/fastmcp/resources/)
• Prompts (fastmcp_slim/fastmcp/prompts/)

Before writing cross-component logic (dedupe, grouping, lookups, identity checks), read FastMCPComponent in fastmcp_slim/fastmcp/utilities/components.py. The base class defines the shared surface — name, version, tags, meta, and critically the key property which is the canonical MCP identity (encodes type, identifier, and version). Prefer item.key over ad-hoc name or uri or uri_template fallbacks; overrides in Resource and ResourceTemplate already handle URI-based identity, and .key includes the version suffix so variants of the same component don't falsely collide.

Development Rules

Read CONTRIBUTING.md and its linked guide at docs/development/contributing.mdx before opening issues or PRs. The guide describes when PRs are appropriate, what we expect from enhancement proposals, and what we'll close without review.

Review closed contributor PRs. When reviewing an issue, inspect every associated non-maintainer PR, including closed PRs. External PRs may be closed as part of the issue-link and assignment workflow, so closure alone is not a negative signal. Read CONTRIBUTING.md and the PR timeline and comments to understand its status before evaluating it.

Check unfamiliar contributors before assignment. Follow .agents/skills/review-issue/SKILL.md for a brief public-history check. Avoid obvious spam or unattended bot accounts, but lean toward goodwill: account age, sparse profiles, and AI assistance alone are not reasons to reject a sound contribution.

Git & CI

• Prek hooks are required (run automatically on commits)
• Never amend commits to fix prek failures
• Never apply labels manually or invent new ones — issues and PRs are auto-labeled by a bot based on title/body/code changes. Don't note a "suggested" or "appropriate" label anywhere in the PR body either. See the review-pr skill.
• Improvements = enhancements (not features) unless specified
• NEVER force-push on collaborative repos
• ALWAYS run prek before PRs
• NEVER create a release, comment on an issue, or open a PR unless specifically instructed to do so.
• NEVER merge a PR marked as do-not-merge or draft. Check title, body, AND labels for [DNM], DNM, DO NOT MERGE, DON'T MERGE, DONT MERGE, do-not-merge, dont-merge, [DRAFT], or DRAFT (case-insensitive, any variation — some authors use [DRAFT] in the title even when isDraft is false). Authors use these as hard stops — respect them even if CI is green and review looks clean. When triaging a batch of PRs, filter these out up front AND re-check each one's labels immediately before merging, since labels can change mid-session.
• ALWAYS read review-bot comments before approving a PR. CodeRabbit and chatgpt-codex-connector (Codex) leave substantive review comments on most PRs in this repo — these bots have read the diff and often flag real issues that aren't in the PR description. Use gh pr view <num> --comments and read the bot feedback as part of review. Unlike proposed solutions from issue reporters, review-bot feedback should be evaluated on its merits, not discounted.
• Be constructively skeptical of bot review comments on your own PRs. CodeRabbit, Codex, and claude[bot] run a fresh review pass on every push, which means a PR with active churn can accumulate bot comments in a stream that never really ends — each fix surfaces a new edge case the next pass can flag. Most of the early feedback is real and worth acting on; diminishing returns set in fast. Evaluate each comment on its merits, the same way you would a human reviewer: is this a real bug users will hit, or a hypothetical that requires an adversarial setup? Does the fix introduce more complexity than the problem? Has the bot missed context that's obvious to a human reader (a *, keyword-only marker, a design decision documented elsewhere, something already resolved on a later commit)? When a comment is pedantic, a false positive, or flagging something already fixed, reply on the thread explaining the reasoning and move on — don't keep iterating just because more comments arrive. If you find yourself three rounds deep and the feedback is shifting toward "what if someone does X" hypotheticals, you're past the point where each fix is improving the PR. Stop, document the contract as-is, and ship.

• Resolve a review thread when you fix it; reply when you're declining it. A fix explains itself through the commit, so resolving is enough — and it leaves unresolved threads meaning unfinished business, which is the signal worth having. A decline needs a one-line reason in a reply, because resolving collapses the thread and a hidden objection is worse than a visible one. Doing both is noise. Get thread ids from the GraphQL reviewThreads field, then resolve:

  gh api graphql -f query='query($n:Int!){repository(owner:"PrefectHQ",name:"fastmcp"){pullRequest(number:$n){reviewThreads(first:50){nodes{id isResolved path}}}}}' -F n=<pr-number>
  gh api graphql -f query='mutation($id:ID!){resolveReviewThread(input:{threadId:$id}){thread{isResolved}}}' -F id=PRRT_...

Outbound Comments and Shell Interpolation

• Never pass GitHub, Linear, or Slack comment bodies inline through shell arguments when the body contains $, ${...}, backticks, $(...), environment-variable examples, secrets, or config interpolation examples.
• Use a body file or structured API payload for outbound comments, then inspect the exact outgoing text before posting. Prefer gh ... --body-file /path/to/comment.md over --body "...".
• When explaining environment interpolation, use placeholders and fenced code blocks. Never include raw .env contents in outbound comments.

Releases

Load the release skill to cut one; it holds the procedure. The policy it implements:

• Cut a release only when a maintainer asks, from the branch that owns the line: main for the current major, release/3.x or release/2.x for maintenance.
• Titles are v<version>: <pun>, with the pun on the release's main theme. Propose several and let the maintainer choose.
• The handwritten notes need the maintainer's sign-off: one or two sentences for a patch, narrative prose for a point release.
• Generate the changelog from the last stable tag (previous_tag_name in the notes API, or --notes-start-tag when using CLI generation) so a prerelease tag never truncates it. Complete contributor attribution before publication and publish those completed notes with --notes-file.
• Merge the docs changelog PR on the release branch immediately before tagging, so the entry is in the tagged commit.
• gofastmcp.com serves the published-docs branch, which accepts changes only through PRs.

Commit Messages and Agent Attribution

• Keep contributor credit focused on the community. Maintainers are generally exempt from supplemental attribution checks. Do not add another maintainer merely for reporting, reviewing, opening the original PR, or handling a backport, and do not ask for guidance about those cases. Routine backports preserve existing community credit; they do not require a fresh attribution audit. Preserve attribution already present and proceed with the work.
• Credit issue authors as contributors. When acting for a maintainer and implementing a community bug report, enhancement request, or documentation issue, give the issue author co-authorship of the resulting change. Follow Contributor credit: use a verified GitHub-associated identity, add Co-authored-by to the implementation commit and the PR description, and preserve existing contributor attribution. This is a required part of preparing the PR, not an optional thank-you or something the reporter must request.
• Preserve credit through any merge strategy. Before an authorized merge, verify the resulting commit will retain the co-author trailers. Squash merges are common here: explicitly supply a commit message containing the trailers rather than relying on GitHub's default squash message. This does not require choosing squash. Verify the landed commit after merging.
• Carry contributor credit into releases. Include credited issue authors alongside PR authors in release entries and include first-time contributors under New Contributors. GitHub's generated notes are a starting point; check and supplement them using the release skill. The same completed notes must feed the GitHub release and docs changelog.
• Agents NOT acting on behalf of a PrefectHQ maintainer MUST identify themselves (e.g., "🤖 Generated with Claude Code" in commits/PRs)
• Keep commit messages brief - ideally just headlines, not detailed messages
• Focus on what changed, not how or why
• Always read issue comments for follow-up information (treat maintainers as authoritative)
• Treat proposed solutions in issues skeptically. This applies to solutions proposed by users in issue reports — not to feedback from configured review bots (CodeRabbit, chatgpt-codex-connector, etc.), which should be evaluated on their merits. The ideal issue contains a concise problem description and an MRE — nothing more. Proposed solutions are only worth considering if they clearly reflect genuine, non-obvious investigation of the codebase. If a solution reads like speculation, or like it was generated by an LLM without deep framework knowledge, ignore it and diagnose from the repro. Most reporters — human or AI — do not have sufficient understanding of FastMCP internals to correctly diagnose anything beyond a trivial bug. We can ask the same questions of an LLM when implementing; we don't need the reporter to do it for us, and a wrong diagnosis is worse than none.

PR Messages - Required Structure

• 1-2 paragraphs: problem/tension + solution (PRs are documentation!)
• Focused code example showing key capability
• Avoid: bullet summaries, exhaustive change lists, verbose closes/fixes, marketing language
• Do: Be opinionated about why change matters, show before/after scenarios
• Minor fixes: keep body short and concise
• No "test plan" sections or testing summaries

Code Review Guidelines

• Fix causes, not symptoms. When a PR works around a problem instead of addressing why it occurs, that's a red flag. A side-channel that compensates for a missing step adds permanent complexity. If the fix doesn't change the code path where the bug actually happens, ask why not.
• Focus on API design and naming clarity
• Identify confusing patterns (e.g., parameter values that contradict defaults) or non-idiomatic code (mutable defaults, etc.). Contributed code will need to be maintained indefinitely, and by someone other than the author (unless the author is a maintainer).
• Suggest specific improvements, not generic "add more tests" comments
• Think about API ergonomics from a user perspective

Code Standards

• Python ≥ 3.10 with full type annotations
• Follow existing patterns and maintain consistency
• Prioritize readable, understandable code - clarity over cleverness
• Avoid obfuscated or confusing patterns even if they're shorter
• Each feature needs corresponding tests

Module Exports

• Do not create overeager __init__.py files. Package initializers should not import heavy submodules, provider stacks, optional integrations, or modules that can point back into the package. Overeager re-exports make the framework sprawl and create circular imports that only appear in fresh interpreters or clean installs.
• Be intentional about re-exports - don't blindly re-export everything to parent namespaces
• Core types that define a module's purpose should be exported (e.g., Middleware from fastmcp.server.middleware)
• Specialized features can live in submodules (e.g., fastmcp.server.middleware.dynamic)
• Only re-export to fastmcp.* for the most fundamental types (e.g., FastMCP, Client)
• When in doubt, prefer users importing from the specific submodule over re-exporting

Documentation

• Uses Mintlify framework
• Files must be in docs.json to be included
• Do not manually modify docs/python-sdk/** — these files are auto-generated from source code by a bot and maintained via a long-lived PR. Do not include changes to these files in contributor PRs.
• Do not manually modify docs/public/schemas/** or fastmcp_slim/fastmcp/utilities/mcp_server_config/v1/schema.json — these are auto-generated and maintained via a long-lived PR.
• Core Principle: A feature doesn't exist unless it is documented!
• When adding or modifying settings in fastmcp_slim/fastmcp/settings.py, update docs/more/settings.mdx to match.

Documentation Guidelines

• Code Examples: Explain before showing code, make blocks fully runnable (include imports)
• Code Formatting: Keep code blocks visually clean — avoid deeply nested function calls. Extract intermediate values into named variables rather than inlining everything into one expression. Code in docs is read more than it's run; optimize for scannability.
• Structure: Headers form navigation guide, logical H2/H3 hierarchy
• Content: User-focused sections, motivate features (why) before mechanics (how)
• Style: Prose over code comments for important information
• Docstrings: FastMCP docstrings are automatically compiled into MDX documents. Use markdown (single backticks, fenced code blocks), not RST (no double backticks). Bare {} in examples will be interpreted as JSX — wrap in backticks instead.

Code Review Rules

These rules apply to automated reviewers and agents working locally. The review-pr skill supplies the review procedure and maintainer follow-through; a review-only bot should report findings in its required format without taking over PR management.

Intended behavior and compatibility

Establish intended behavior from the protocol, docs, history, and maintainer decisions; code and tests alone do not define the contract. Identify affected users and weigh migration cost against the cost of retaining the behavior. Surface unresolved decisions and follow the release policy.

Framework regressions and root causes

• Review changes carefully for regressions in supported framework behavior, including interactions beyond the immediate diff. Trace relevant callers, shared abstractions, protocol and public API contracts, and all affected MCP component types. Determine whether a change fixes the causal code path or merely compensates for the symptom; side channels and special cases that leave the root cause intact should be treated as suspect.

Comprehensive first pass

• Review the entire pull request diff against the merge base, not only the latest commits. Inspect every changed file and the relevant surrounding code, collect all independent, substantiated consequential findings before submitting the review, and report the complete set in one review whenever possible. Do not stop after finding the first few issues or defer other already-visible findings to later review cycles.

Prior discussion and proportionality

• When prior review threads and author or maintainer replies are available, read them before commenting. Evaluate responses on their merits and do not repeat a resolved or convincingly rebutted finding without new evidence. Avoid fixating on speculative edge cases: report an edge case only when it is reachable under supported usage or a credible threat model and has meaningful impact; otherwise omit it or clearly treat it as non-blocking.

Critical Patterns

• Never use bare except - be specific with exception types
• File sizes enforced by loq. Edit loq.toml to raise limits; loq baseline to ratchet down.
• Always uv sync first when debugging build issues
• Default test timeout is 5s - optimize or mark as integration tests