* Studio: wire Anthropic server-side context compaction
Anthropic ships server-side context compaction as a beta
(`compact-2026-01-12`). When the rendered prompt crosses the
configured input-token threshold, Anthropic runs an extra LLM pass
that summarises older turns and the request continues against the
compacted prefix. The response carries the original top-level fields
plus a new `context_management` block (with `applied_edits`) and
`usage.iterations[]` accounting per pass.
Per the docs the feature is currently supported on Opus 4.6, Opus 4.7,
Sonnet 4.6, and Mythos preview. The minimum threshold is 50k tokens;
under-50k requests 400.
Changes:
- Add prefix gate + helper `_anthropic_supports_compaction` plus
constants `_ANTHROPIC_COMPACTION_PREFIXES`, `_ANTHROPIC_COMPACTION_BETA`,
`_ANTHROPIC_COMPACTION_TYPE`, `_ANTHROPIC_COMPACTION_MIN`.
- Add `compaction_threshold: Optional[int]` to ChatCompletionRequest
(50k ge bound, 2M le bound). Thread through `routes/inference.py`
-> `stream_chat_completion` -> `_stream_anthropic`.
- In `_stream_anthropic`, when threshold is set AND the model
accepts compaction, attach `context_management.edits[{type:
"compact_20260112", trigger:{type:"input_tokens", value:N}}]` to
the outbound body. Sub-50k values are clamped up to 50k to keep
the request well-formed.
- Refactor the anthropic-beta header builder to merge any combination
of `code-execution-2025-08-25` + `compact-2026-01-12` flags into
one header value. Unrelated betas added at the registry level still
pass through.
- Add `test_anthropic_compaction.py` with 16 cases: gate matrix
(every doc-listed model), correct body shape, threshold clamping,
beta header merge with code execution, silent no-op on unsupported
models, omitted-threshold pass-through.
Live verified end-to-end against the real Anthropic API:
`compact_20260112` accepted on Opus 4.7, response carries
`context_management.applied_edits` + `usage.iterations[]` as
documented. (The first WebFetch-summarised version of these docs
suggested `compact_20260120`; the actual API only accepts
`compact_20260112`, matching the beta-header date. Worth pinning
behind a test so a future doc update can't drift back.)
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* Address review: drop ge=50_000 clamp + parse usage.iterations[]
Two reviewer follow-ups on the compaction PR:
1. Pydantic ge=50_000 on compaction_threshold was dead code.
FastAPI rejected sub-50k threshold values with a 422 before the
`max(int(...), _ANTHROPIC_COMPACTION_MIN)` clamp in
_stream_anthropic could ever fire. Relaxed the floor to ge=1 so
the in-helper clamp actually does its job; the schema comment
now explains why this is intentional. Added a regression test
that posts a value of 1 and 49_999 through the real request
schema.
2. Anthropic publishes per-iteration token counts in
`usage.iterations[]` whenever a fresh compaction has run, and
the top-level input_tokens / output_tokens cover only the
`message` iteration -- billing must add the compaction
iterations on top. Aggregate compaction iteration tokens into
`last_usage["compaction_input_tokens" / "compaction_output_tokens"]`
so the cost surface (PR 5690) can read them without re-walking
the array, and surface both figures in the closing stream
summary log. Added two tests: one that pins the aggregation on a
compacted turn and one that pins `None` when no fresh
iterations land (so re-applied compaction blocks don't double-bill).
Sourcing: https://platform.claude.com/docs/en/build-with-claude/compaction
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* Address review: round-trip Anthropic compaction blocks across turns
Codex P1: once context_management is enabled and Anthropic runs
server-side compaction mid-stream, the response carries a
`{type:"compaction", content:"<summary>"}` content block on the
assistant message. The translator only handled text_delta and
input_json_delta on content_block_delta, so the compaction block
was silently dropped. Worse, the request schema's ContentPart
discriminated Union didn't accept `type:"compaction"`, and
_build_external_messages didn't pass it through, so even a
hand-crafted assistant message carrying the block would 422 at
parse time. Net result: Anthropic re-compacted from scratch on
every subsequent turn, wasting input tokens and reasoning budget.
End-to-end backend wiring of the round-trip:
1. SSE translator. _stream_anthropic now tracks a `current_compaction`
state slot. content_block_start with type=="compaction" seeds it
(Anthropic may include the summary on the start event AND/OR
stream it via text_delta events on the same block index --
handle both). text_delta inside a compaction block routes into
the compaction buffer instead of the user-visible content
stream, since the summary is opaque internal state, not
assistant prose. content_block_stop emits a `compaction_block`
tool_event carrying the full summary so the chat-adapter can
persist it. compaction_blocks_seen is surfaced in the closing
summary log.
2. Pydantic schema. Added CompactionContentPart with Tag("compaction")
on the ContentPart Union so requests carrying the block parse
cleanly. Required `content` field with a docstring pointing at
the Anthropic docs.
3. Message builder. _build_external_messages forwards compaction
parts on both vision and non-vision paths; the per-provider
stream helper decides whether to forward to the wire (Anthropic
does; other providers ignore the part). When a non-vision route
ends up with a single text part, collapse back to a string
so providers that don't accept content arrays still get the
expected shape.
4. _stream_anthropic outbound translator. {type:"compaction"} parts
on an assistant message land on the wire verbatim. Empty/missing
`content` is skipped so a malformed stored block can't 400
Anthropic.
Tests added (5): stream emits compaction_block tool event with the
summary intact; user-visible content stream does NOT carry the
summary text; outbound body forwards compaction parts verbatim on
the next turn; Pydantic schema accepts the part; builder passes
it through on both vision and non-vision provider routes.
Frontend follow-up: the chat-adapter needs to persist the
compaction_block tool_event onto the stored assistant message so
turn N+1 includes it in payload.messages. Pinned in the PR
description.
Sourcing: https://platform.claude.com/docs/en/build-with-claude/compaction
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* Address review: gate compaction-part passthrough to Anthropic only
Codex P1: my previous round-trip change preserved {type:"compaction"}
parts on every provider route in _build_external_messages. That
meant a chat history with prior compaction state silently leaked
the Anthropic-specific block to OpenAI/DeepSeek/Mistral/Gemini/
Kimi/OpenRouter on a provider switch, where generic
/chat/completions passthrough hands the unknown content type to
the upstream API and 400s the whole turn.
Added a `provider_type` kwarg to _build_external_messages and
gated the compaction forwarder on `provider_type == "anthropic"`.
Every other value (including the legacy None for callers that
don't pass it yet) strips the part. The Anthropic stream helper
still maps it to a native `compaction` block on the wire.
Threaded provider_type through from _proxy_to_external_provider's
call site.
Tests updated: vision + provider="anthropic" still forwards; six
non-anthropic providers strip the part; missing provider_type
strips defensively; non-vision + anthropic still forwards; non-vision
+ non-anthropic collapses back to a text string.
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