Reddit user Temporary_Method6365 reports Sonnet 5.5 buffers all output after a tool_result until message completion — reproduced across the Anthropic API, Bedrock, and OpenRouter with repro and data filed as anthropic-sdk-python issue #1960 — and Anthropic's fix or acknowledgment, or refutation of the repro, settles whether this is a provider-side streaming regression that streaming agent harnesses must work around.
state: seedheat: lowuncertainty: mediumnovelscott: highanthropic sonnet-5-5 api-reliability agent-harnessesAnthropic
What is this?
A Reddit report claims Claude Sonnet 5.5 (model ID claude-sonnet-5-5, released Sept 28, 2026 and made Claude Code's default Sonnet the same day) buffers all streamed output after a tool_result until message completion, said to be reproduced across the Anthropic API, Bedrock, and OpenRouter and filed as anthropic-sdk-python issue #1960. The supplied search results do not surface that SDK issue or any direct corroboration of the buffering claim — the repro, the issue number, and the cross-provider confirmation all rest on the report itself. They do show an active cluster of adjacent Sonnet tool-turn stream failures: oh-my-pi issue #2685 has Sonnet via OpenRouter's openai-completions path aborting after the first tool call with 0 tokens, there attributed to the adapter rather than Anthropic's own responses, and Anthropic has documented Sonnet 5.5 cross-provider breaking changes (thinking-block replay returning 400s on Claude API/Bedrock/Google Cloud, forced tool_choice 400s). So the environment is one of genuine Sonnet 5.5 tool/streaming friction across providers, but this specific buffering claim is unconfirmed, and the oh-my-pi case is a caution that a cross-provider symptom can still turn out to be a shared compat-adapter bug rather than a provider-side regression.
Why it matters to Scott
Suspected model-level streaming regression sitting on the exact seam of Scott's active builds: ask's tool-result streaming path, the LiteLLM gateway his projects route through, and Claude Code — where Sonnet 5.5 is now the default — all sit downstream of it, and a fault spanning API/Bedrock/OpenRouter is precisely the case his provider-level fallback paths cannot route around, stress-testing what task-aware model routing assumes. Unconfirmed pending Anthropic's response to #1960 (the oh-my-pi adapter caution applies), but either resolution — fix or refutation — changes what he pins, tests, or builds a workaround for.
dev:project.askdev:technology.litellmdev:concept.task-aware-model-routingdev:technology.claude-coderadar:anthropic-opus55-prompting-guideradar:concept.anthropicradar:concept.agent-harnessesradar:concept.tool-callingradar:concept.model-regressionradar:concept.claude-code
queries asked of Scott's wikis
- agent harness streaming tool result handling
- multi-provider fallback OpenAI-compatible adapter
- model upgrade regression testing checklist
- LLM provider reliability retry fallback patterns
- anthropic sdk integration quirks notes
Measured heat
now 0 pts/hpeak 1 pts/hcomments 0/hpeers p0momentum: steady1 platformsage 273h
points/hour across evidence · reading as of 2026-10-12 02:59:37.977291+11:00 · deterministic, not a model opinion
How the heat travelled
pace: p35 vs 1188 stories at the 168h mark (now 273h old) — ahead of agentgate-signed-agent-receipts (1.3x), behind agentic-determinism-index (0.8x)
Evidence (1) — ⭐ canonical anchor
Interpretation history
2026-09-30T08:45:23Z
grounded: novel/high — Suspected model-level streaming regression sitting on the exact seam of Scott's active builds: ask's tool-result streaming path, the LiteLLM gateway his project
2026-09-30T08:37:12Z
case created — Cross-provider reproducible regression claim anchored to a filed SDK issue — exactly the plumbing agent-harness builders need tracked, distinct from the existing Sonnet 5.5 token-economics case.
Decision trace
- 09-30 18:45groundSuspected model-level streaming regression sitting on the exact seam of Scott's active builds: ask's tool-result streaming path, the LiteLLM gateway his projects route through, and Claude Co
- 09-30 18:37createCross-provider reproducible regression claim anchored to a filed SDK issue — exactly the plumbing agent-harness builders need tracked, distinct from the existing Sonnet 5.5 token-economics case.