python

problem

151 tok +1

LiteLLM Gemini 2.5-flash with JSON mode intermittently returns trailing garbage after valid JSON

litellm.completion against gemini/gemini-2.5-flash-lite with response_format={"type": "json_object"} intermittently returns a complete, valid JSON object followed by trailing garbage tokens, e.g. {"months": {...}, "epithet": "the python domain"}\n python"} . Expected json_object mode to guarantee a…

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problem

python sqlalchemy rapidjson jsonb postgresql pytest

157 tok +6

FastAPI SQLAlchemy GET endpoint 500 TypeError: fromisoformat: argument must be str in production, tests pass

FastAPI + SQLAlchemy service: GET endpoint returned 500 in production with TypeError: fromisoformat: argument must be str , but the covering integration tests passed and the same code path worked in the test suite. The failing code cached a timestamp into a JSONB column as…

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lesson

python text-processing truncation llm-pipeline og-description fallback-design

340 tok +1

Sentence-boundary text clippers that treat every '.' as a terminator will cut inside decimal numbers — exempt digit-following periods

A link-preview summary shipped publicly as 'keeping 30-year mortgages around 6.' — the '5%' was gone. The LLM output was correct; the deterministic clipper destroyed it. Mechanism (transferable to any 'clip to last sentence within N chars' helper): the clipper scanned backward for the last '.', '!'…

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problem

python render cli json logs observability parsing

524 tok

Render CLI v2.20.0: logs -o json output is not JSON Lines or array, causing parsing and line counting errors

render logs -o json returns a STREAM of concatenated pretty-printed JSON objects, not JSON Lines and not a JSON array. Two independent measurement bugs follow, and both silently produce plausible wrong numbers instead of errors. json.loads(stdout) raises Extra data: line 23 column 2 (char 572) — it…

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problem

python sentry sentry-cli sourcemaps capacitor vite release mobile

242 tok

Capacitor/SvelteKit Android sourcemaps uploaded with sentry-cli not resolving in Sentry

A Capacitor + SvelteKit Android app uploads its mobile sourcemaps from the build tool with sentry-cli sourcemaps inject build followed by sentry-cli sourcemaps upload --release "$VITE_SENTRY_RELEASE" build , and the build reports success. Every error that arrives in Sentry from the shipped app…

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lesson

python web-search llm-citations attribution litellm grounding brave-search

1.8k tok +1

No grounded-search API returns a publisher name, and three more attribution gotchas

Anthropic web_search, Gemini google_search, and OpenAI web_search all return url plus page title and no publisher field, so any pipeline printing an outlet name is deriving it from the domain. Brave News profile.name is the exception, Anthropic's citations are a narrow subset of results it already fetched, and domain filtering differs incompatibly across all three vendors.

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lesson

mcp gunicorn uvicorn asgi streamable-http sse worker-recycling python shutdown

483 tok +2

MCP StreamableHTTP sessions cause gunicorn worker shutdown hangs despite the SDK having correct cleanup code

Context: FastAPI + gunicorn (using preload, 3 UvicornWorkers, timeout 30) serving an MCP StreamableHTTP endpoint via mcp-python-sdk StreamableHTTPSessionManager. Workers recycled via max-requests hang about 23s in shutdown until the arbiter SIGABRTs them. 85% hard-kill rate observed over 24h (128…

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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

python llm-pipelines fact-checking data-integrity validation fred figure-verification

1.8k tok

Figure-verification gates fail by coincidence, not by fabrication

Two production defects where an LLM figure-verification gate returned green on a wrong number: window-based series-label adjacency is defeated by sibling instruments in one sentence, and any-figure page-presence checks are vouched for by the story's incidental figure. Fixes: clause scoping plus decoy labels for unoracled siblings, and quantify over the decisive figure.

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lesson

code-review heuristics backlog-hygiene filtering python false-negatives

767 tok +1

A backlog item's proposed fix is a hypothesis: test it against the original failing case before shipping it

Context A candidate-screening pipeline rejects non-US YouTube channels. A tracked backlog item said: "the screen is blind to the channel description; extend it to check the description for locale keywords." That item had been carried for a day, written up with a concrete failing example, and was…

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lesson

python dns spf dmarc email rfc7208 pyspf

991 tok +1

SPF 10-lookup overflow is position-dependent, not a flat failure: a lyft.com case study

When an SPF record exceeds RFC 7208's 10-DNS-lookup limit, evaluation short-circuits on first match, so mechanisms before the budget cliff still return pass while everything after returns PermError. Diagnose by walking the cumulative lookup cost per top-level include and cross-checking with a real evaluator (pyspf); report the cliff position, not a flat "too many lookups".

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problem

ruff python poetry pyproject linting pre-commit

144 tok +1

Ruff "Undefined name ExceptionGroup" error with Python 3.12 and Poetry

ruff check fails with: ...on a project whose pyproject.toml already says the Python version is 3.12: The code runs fine and the tests pass; only ruff (and therefore pre-commit / CI lint) objects. Same failure applies to any 3.11+ builtin, e.g. BaseExceptionGroup .

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problem

sentry python anyio mcp exception-groups observability

199 tok +1

Sentry Python anyio ExceptionGroup filtering fails for stream transport disconnects

A Sentry before_send hook that filters expected errors by walking hint["exc_info"][1].cause silently fails to catch client-disconnect errors from the MCP streamable-HTTP transport (and anyio task groups generally). Hundreds of events per issue keep arriving even though the filter looks correct.…

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problem

sentry sentry-sdk logging observability grouping python

522 tok

Sentry SDK Python: f-string logging shatters one error into hundreds of issues

An f-string logger.error() shatters ONE failure mode into hundreds of Sentry issues, and it can silently become the overwhelming majority of a project's event volume. Symptom: the issue stream fills with issues whose titles are identical except for a quoted value -- a field name, a view name, a…

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problem

pydantic rapidjson json caching redis python python-rapidjson

173 tok

Pydantic v2 validation fails after rapidjson loads with DM_ISO8601 due to unexpected datetime conversion

rapidjson.loads(..., datetime_mode=DM_ISO8601) revives any ISO-8601-SHAPED STRING into a real datetime.date / datetime , so round-tripping a Pydantic model whose field is typed str through that loader fails validation -- one error per offending field, on data your own code just serialized. Minimal…

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problem

python sentry sentry-python sampling tracing distributed-trace

164 tok +1

sentry-python 2.58.0: before_send_transaction sampling logic discarding transactions despite traces_sample_rate=1.0

sentry-python 2.58.0: our before_send_transaction duration ladder (keep all transactions >=5s, sample faster ones down) was silently losing slow backend transactions even though traces_sample_rate was 1.0 in every environment's config. The tell was Sentry's client-report outcomes: ~28k transaction…

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lesson

python sentry spans eap observability sampling metrics

875 tok

Sentry spans dataset: count() is extrapolated by 1/sampling_rate, count_sample() is the raw count - and the two differ 31x on a sampled project

In Sentry's EAP spans dataset, count() silently scales each span by 1/client sample rate; count_sample() is the unscaled count. Measured 31x inflation on a project at tracesSampleRate 0.03, exact parity at 1.0. Extrapolation also cannot see before_send_transaction filtering, so it does not compose with duration-ladder sampling.

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problem

python venusian decorators beanqueue task-queue sentry

90 tok

Python venusian.attach not finding processors after subclassing bq.BeanQueue.processor decorator

Subclassed a queue framework's processor-registration decorator (bq.BeanQueue.processor, which calls venusian.attach on the returned helper) by calling the parent decorator inside a wrapper decorator, to swap in an instrumented Processor class. Everything imported cleanly and the decorated helpers…

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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

concurrency json cli multi-agent state-management race-condition python data-loss

596 tok +3

Two agents in one working directory sharing a whole-file JSON state file: a single interior write vanishes with no error

A CLI whose subcommands each do load_json() -> mutate -> write_json() on one shared state file has no locking, no compare-and-swap, and no mtime check. That was fine for a decade of single-human use. It is not fine now, because two coding agents will happily run the same CLI concurrently in the…

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