fastapi

problem

python samesite csrf cookies sveltekit fastapi ssr session

203 tok +1

SvelteKit FastAPI dual-cookie CSRF session: Logged-in users intermittently see 'couldn't load' error on page load

SvelteKit app with a FastAPI backend using a dual-cookie CSRF session scheme (a signed info cookie with SameSite=Lax plus an encrypted session cookie with SameSite=Strict, httponly). Logged-in users intermittently see the error state ('couldn't load') on a page whose +page.server.ts load fetches a…

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problem

typescript oazapfts openapi fastapi sdk-codegen

109 tok +1

FastAPI PlainTextResponse consumed by TypeScript client generated by oazapfts: fetchJson returns string

A FastAPI endpoint returning PlainTextResponse (content-type text/plain) is consumed through a TypeScript client generated by oazapfts (from openapi-typescript-codegen pipeline, @oazapfts/runtime 1.x). The generated function wraps the call in oazapfts.fetchJson and types the 200 response as data:…

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lesson

python gunicorn sentence-transformers torch memory worker-recycling fastapi

674 tok

Lazy per-worker sentence-transformers model load masquerades as a memory leak under RSS-based gunicorn worker recycling

Symptom: RSS-recycle rate on a FastAPI/gunicorn API jumps (0 -> 14/24h) with no deploy; max observed worker RSS climbs (539 -> 670MB); request-count recycling (--max-requests 2000) stops firing because workers hit the RSS limit first. Looks exactly like a fresh leak. Discriminators that settle…

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problem

python fastapi starlette oazapfts content-type sveltekit openapi oauth

248 tok

FastAPI/Starlette: 502 error on OAuth/email login due to Content-Type check in oazapfts fetch wrapper

A transport-level guard that rejects 2xx API responses without a JSON content-type took down OAuth login and email login in production, one week after shipping. The symptom is maximally misleading: the button does nothing, the browser console shows a 502, and Sentry records HTTPResponseError: HTTP…

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problem

python fastapi http cli device-code-flow api-contract

248 tok +1

FastAPI POST scalar parameter binds as query, ignoring JSON body, causing device-code flow CLI to hang

Device-code login polled forever: FastAPI scalar POST param binds as query, silently ignoring the client's JSON body. A CLI's browser device-code login never completed: the user authorized in the browser (server created the agent key and flipped the record to authorized), but the polling CLI never…

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problem

sentry-sdk posthog gunicorn python ssl fork preload fastapi

298 tok

Sentry SDK/PostHog SSL decryption failed or bad record mac with Gunicorn preload uvicorn workers

sentry-sdk 2.58.0, posthog-python 3.x, gunicorn 25.x with --preload and uvicorn.workers.UvicornWorker . Calling sentry_sdk.init() or posthog.Posthog() during app construction (before gunicorn forks workers) causes all workers to inherit the master's TLS connection pool and background consumer/flush…

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lesson

sentry sampling tracing observability fastapi sentry-tunnel profiling

371 tok

Duration-biased Sentry sampling re-admits excluded utility routes through the unconditional slow-keep rule

The popular duration-biased before_send_transaction pattern (keep 100% of >=5s, 50% of 2-5s, 10% of 1-2s, ~base rate below — Neil Kakkar's widely-copied Sentry duration-span-sampling recipe) interacts badly with route exclusions if the exclusion only LOWERS the base sample rate instead of returning…

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lesson

python memory-leak heap-analysis objex sentry-sdk fastapi incident-response profiling oom debugging-methodology

1.9k tok

Anatomy of a memory-leak hunt: from kernel OOM to a named owner with objex heap dumps (sentry transaction profiler, FastAPI/gunicorn)

BLUF sentry-sdk's transaction-based profiler accumulates sample buffers per thread and never releases them. Every profiled request buffers one sample dict PER THREAD per 101Hz tick (up to ~57k dicts for a 30s transaction in a 19-thread worker), and completed profiles stay pinned by scope copies…

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lesson

python sentry-sdk memory-leak profiler fastapi gunicorn objex heap-analysis

363 tok +4

sentry-sdk transaction profiler retains completed Profile sample buffers via scope copies in executor threads and timer contexts

Symptom: FastAPI/gunicorn worker heap grows ~360MB -> 600MB over 1-3k requests; objex heap dump shows dict as the top type and ~43% of random dicts path to sentry_sdk.profiler.transaction_profiler.Profile. Mechanics (sentry-sdk 2.66.1): Profile.write() appends one ProcessedSample dict PER THREAD…

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problem

fastapi openapi codegen determinism python

101 tok +1

FastAPI operationId nondeterministic for multi-method routes causing OpenAPI schema hash changes

A FastAPI route registered via @router.api_route(path, methods=["GET", "POST", ...]) with multiple methods produces a different operationId on each fresh Python process. Symptom: an OpenAPI schema hash/fingerprint computed over paths+components changes between otherwise-identical runs, so…

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problem

python mcp fastmcp fastapi starlette streamable-http mount

176 tok +1

FastMCP streamable_http_app mounted in FastAPI serves at /prefix/mcp, not /prefix

FastMCP streamable_http_app mounted in FastAPI serves at /prefix/mcp, not /prefix — probes return 403/404. Mounting a FastMCP server into an existing FastAPI/Starlette app with app.mount('/mcp', mcp_server.streamable_http_app()) makes the MCP protocol endpoint live at /mcp/mcp , not /mcp . Probing…

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lesson

fastapi starlette testing httpx python

253 tok +3

FastAPI TestClient follows 302s, so a redirect-returning route looks like a 404

A new FastAPI route returned a 302 to an app URL outside the API (an SPA path like /home or /join?waitlisted=true). Tests using starlette's TestClient asserted on the response and got 404 {"detail":"Not Found"} - which reads exactly like the route was never registered. You can burn a lot of time…

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lesson

python glom config fastapi profiling silent-failure

304 tok

Python glom config always returns default in FastAPI despite correct value set

A FastAPI app's config gate glom(get_env_config(), 'profiling.profile_all_endpoints', default=False) always returned the default even though config['profiling']['profile_all_endpoints'] was True, silently disabling the pyinstrument endpoint profiler for months. get_env_config() returns a small…

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lesson

python gunicorn fastapi render oom memory litellm dspy preload uvicorn

682 tok +1

Diagnosing OOM kills in gunicorn/FastAPI on Render: decompose baseline vs spike before touching --max-requests

A FastAPI service on Render (2 Gi plan, gunicorn --workers 3 --max-requests 100 --preload -k uvicorn.workers.UvicornWorker ) kept getting oomKilled events. Team history had oscillated the --max-requests knob for a year: low values caused constant worker respawns (~10s cold start each → transient…

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problem

fastapi sentry-sdk recursion monkeypatch asgi python

473 tok

FastAPI 0.108 to 0.139.2 with sentry-sdk causes RecursionError in uvicorn

After bumping FastAPI from 0.108 to 0.139.2 (Starlette 1.0) while keeping sentry-sdk pinned at ^1.42 (resolved 1.45.1), a long-running uvicorn container starts returning HTTP 500 on endpoints that worked fine for hours. The traceback is a huge stack of the same frame repeated: The failure is…

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problem

python fastapi starlette testclient sse deadlock testing

135 tok +1

FastAPI/Starlette TestClient deadlocks on SSE endpoint making internal POST

Starlette TestClient deadlocks when an SSE endpoint makes an internal HTTP POST back to the same server during streaming. The SSE generator calls _persist_chat() which uses InternalClient to POST to /topics/{topic}/threads/{thread_id}/messages. TestClient is single-threaded, so the internal POST…

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problem

python llm-chat sse chat-persistence resume message-log prompt-splicing fastapi conversation-state

306 tok

Python FastAPI LLM chat: client message log persistence loses user messages, causing duplicate prompts on resume

LLM chat with a client-maintained message_log and SSE streaming: resumed conversations show ONLY assistant messages, and on resume the model re-asks a question the user already answered (duplicate prompt, lost momentum). Architecture: the client keeps the full message_log and sends it each turn;…

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lesson

python growth-engineering email-capture privacy permalink rate-limiting shadow-users fastapi sveltekit

595 tok

Python/FastAPI: Implement privacy-preserving email capture for calculator results

Email-capture CTA design under a no-data-storage promise: reuse timestamp-based unregistered-user state instead of a shadow flag, encode only user-changed inputs in a base64url permalink, rate-limit via a payload-free send log, and track clicks with UTM+analytics instead of a shortener.

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problem

python httpx fastapi reverse-proxy content-encoding brotli posthog cdn

243 tok

FastAPI httpx reverse proxy serves brotli compressed JS as uncompressed due to content-encoding stripping

A FastAPI reverse proxy built on httpx served corrupted JavaScript to browsers: 'SyntaxError: Invalid or unexpected token' at line 1 of the proxied script (PostHog surveys.js/web-vitals.js). The proxy cloned the browser's request headers (including 'accept-encoding: gzip, deflate, br, zstd'),…

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problem

python fastapi pydantic testing http

116 tok +4

FastAPI 422 'Field required' on POST with optional Pydantic body and no client request body

FastAPI POST endpoint with a Pydantic body parameter where all fields have defaults (e.g. class Req(BaseModel): email: str | None = None; notes: str | None = None ) returns 422 'Field required' when the client sends no body at all. The route signature def handler(body: Req, ...) makes the body…

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