[aheritier]

Part of #4654 (PR 2 of 3). Part of #4583 and epic #4580.

What

Two compaction fixes, then two end-to-end tests that pin the whole compaction lifecycle on Claude 5.x: compaction, a SQLite reload, and the next request.

Fixes

Commit	Bug	Fix
fix(anthropic): replay native compaction only where it is enabled	The replay check compared only the provider-state name, which the Claude API and Vertex clients share (anthropic). After a native compaction on the Claude API, a client without native_compaction (Vertex, or a Claude API model without the option) still sent the signed block and the compact-2026-09-04 beta.	A client replays the block only when its own provider_opts.native_compaction is set. Otherwise it sends the summary text that every summary message already carries.
fix(runtime): price native compaction iterations separately	The compaction cost was computed on the summed usage, so a long-context tier was chosen on the total, not per request.	chat.CompactionResult.Iterations keeps each reported iteration. Each one is priced on its own and the costs are summed. If any iteration can't be priced, the cost is nil, as for any other unpriced call.

Tests

Test	Pins	Mutation that makes it fail
TestCompaction_NotReplayedWithoutNativeCompaction, TestVertexDoesNotReplayCompactionBlock	No block and no beta without the option, on the Claude API and on Vertex	The fix itself (written first, red, then green)
TestNativeCompactionPricesEachIteration, TestCompactionCostUnpricedIteration	Tiered rates chosen per iteration; nil on an unpriced iteration	The fix itself
TestClaude5LocalCompactionLifecycle (pkg/runtime)	Threshold compaction summarizes the long history and keeps the signed thinking turn and the latest question. After a SQLite reload, the next request carries the summary, that turn's signature, and the drop_block binding	Loading FirstKeptEntry as 0; dropping both ProviderState and ThinkingSignature on load
TestClaude5NativeCompactionLifecycle (pkg/runtime)	Native compaction stores the signed block, both iterations and their cost. After a reload, the next request starts with the block, verbatim, and sends the beta	Forcing the native_compaction gate off

Both end-to-end tests use a real Sonnet 5.5 client built by the provider registry, a small scripted SSE server (anthropictest, ~150 lines), and the SQLite store. Each takes ~0.3s under -race.

Not added, and why

• Per-iteration tier choice end to end: the runtime test uses flat test rates, so it pins that iteration costs reach the stored summary. Choosing a tier per iteration is pinned by the unit test, which is cheaper.
• Bedrock compaction lifecycle: it would need a Bedrock event-stream fake server, which test(runtime): Claude 5.x lifecycle — compaction, reload, switches, shared clients #4654 rules out.
• Switching models and reloading at request level: PR 3.

Notes

• Dropping ProviderState alone doesn't break the local lifecycle test: the Anthropic converter rebuilds the signed thinking block from ReasoningContent + ThinkingSignature. Both have to be lost for the signature to be lost.
• The on-demand compaction page now lists Google Cloud (beta) among its platforms. docker-agent still rejects native_compaction on Vertex; lifting that is out of scope here.

References

Change	Documentation
A request that carries a compaction block needs the compact-2026-09-04 beta header ("Send the compact-2026-09-04 beta header … on every later request that carries the signed block"); without it, a 400 says compaction is not one of the expected content block types	Compaction on demand
Keep the block exactly as returned, including its signature, and send it first on the next request	Compaction on demand
"The summarization call is billed … and usage.iterations reports it … sum across usage.iterations, not the top-level fields"	Compaction on demand
Haiku 5.5 "is priced by prompt length … Each request is priced on its own", so each compaction iteration picks its own tier	Claude pricing
A kept thinking block is sent back with its signature; thinking.block_binding.prefix_mismatch_behavior: "drop_block" drops it, rather than failing, when the prefix changed	Preserved thinking

Validation

• task build passes.
• task test passes.
• golangci-lint v2.14.0 reports 0 issues, and go run ./lint . reports no offenses.
• go test -race -shuffle=on ./pkg/runtime/ ./pkg/model/provider/anthropic/... ./pkg/chat/ passes.
• An independent review found no critical or important issues. Three minor findings are fixed here:
  • a test comment overstated what the flat-rate cost assertion pins;
  • a raw-JSON assertion now counts messages instead;
  • the provider docs no longer say every Anthropic client replays the block.

Deferred minor findings:

• The native_compaction option name is written as a literal in the Anthropic and Vertex packages and as a constant in the runtime.
• The internal Anthropic test helpers (newScriptedServer, textReply, compactionReply) could move onto anthropictest.

[Copilot]

🟢 Approval recommended

No blocking correctness issues remain; validation results were reported but not independently rerun.

1 open issue

• Enable native compaction in edited-summary replay test · New

What changed in this PR

Fixes native compaction replay and billing for Claude clients, with lifecycle coverage for compaction, SQLite reload, and continuation.

Changes:

• Gate signed-block replay on native_compaction.
• Preserve and price compaction usage per iteration.
• Add provider regression tests and runtime lifecycle tests.

File	Description
pkg/​runtime/​native_compaction.go	Prices iterations separately.
pkg/​runtime/​native_compaction_test.go	Tests tiered and unpriced costs.
pkg/​runtime/​claude5_compaction_lifecycle_test.go	Tests compaction, reload, and continuation.
pkg/​model/​provider/​anthropic/​vertex/​vertex_test.go	Tests summary fallback on Vertex.
pkg/​model/​provider/​anthropic/​features_integration_test.go	Enables compaction in replay coverage.
pkg/​model/​provider/​anthropic/​compaction.go	Gates replay and captures iterations.
pkg/​model/​provider/​anthropic/​compaction_test.go	Tests replay gating and iteration usage.
pkg/​model/​provider/​anthropic/​client.go	Uses client-specific replay eligibility.
pkg/​model/​provider/​anthropic/​beta_converter.go	Applies the replay gate during conversion.
pkg/​model/​provider/​anthropic/​beta_client.go	Gates the compaction beta header.
pkg/​model/​provider/​anthropic/​anthropictest/​server.go	Adds scripted responses and request capture.
pkg/​model/​provider/​anthropic/​anthropictest/​server_test.go	Tests the scripted server.
pkg/​chat/​compaction.go	Stores and clones iteration usage.
docs/​providers/​anthropic/​index.md	Clarifies replay eligibility.

_{Review effort: Balanced}