[aheritier]

Follow-up to #4666 (part of #4653): addresses the Copilot review comment #4666 (comment), which arrived as #4666 entered the merge queue.

What

The Bedrock native-block recorder appended each streamed text or reasoning delta to an immutable string, copying the whole block on every delta: quadratic in response length, on every Bedrock stream. Deltas now accumulate in a per-block strings.Builder, materialized once in state().

Test

TestBlockRecorder_AccumulatesDeltasWithoutPerDeltaCopies records 1000 reasoning plus 1000 text deltas and asserts allocations stay below n/10. Before the change it measured 2002 allocations, one per delta. It also checks the rebuilt blocks.

References

Change	Documentation
strings.Builder minimizes memory copying when building a string incrementally	Go: strings.Builder

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/model/provider/... ./pkg/session/ ./pkg/chat/ passes.

All of this ran on the identical change before rebasing onto main; after rebasing, the Bedrock tests and lint were rerun.

[Copilot]

🟢 Approval recommended

The focused optimization preserves recording behavior, includes regression coverage, and has no unresolved findings.

No open issues 🎉

What changed in this PR

Optimizes Bedrock native-block recording by replacing repeated string concatenation with per-block builders, preserving captured content for replay.

Changes:

• Accumulates text and reasoning deltas in strings.Builder.
• Adds regression coverage for allocation counts and serialized block contents.

File	Description
pkg/​model/​provider/​bedrock/​native_blocks.go	Buffers deltas and materializes text in state().
pkg/​model/​provider/​bedrock/​native_blocks_test.go	Checks allocation limits and reconstructed blocks.

_{Review effort: Balanced}