Populate ResponseUpdate.FinishReason for the Anthropic provider from stop_reason

[PratikDhanave (PratikDhanave)]

What

The Anthropic provider never populated ResponseUpdate.FinishReason, so collected responses had an empty FinishReason even though the Messages API returns stop_reason on both the non-streaming response and the streaming message_delta event.

This adds a package-level mapStopReason helper that maps the SDK stop_reason to the canonical FinishReason values shared across providers:

• end_turn, stop_sequence, pause_turn -> stop

• max_tokens -> length

• tool_use -> tool_calls

• refusal -> content_filter

• empty / unknown -> ""

• Non-streaming: sets FinishReason: mapStopReason(resp.StopReason) on the update.

• Streaming: captures the stop reason from MessageDeltaEvent and sets it on the terminal update, guarding against a later empty chunk overwriting a value already captured.

Why

Brings the Anthropic provider to parity with the other providers, which already surface a finish reason: openaiprovider/chat.go sets finishReason = choice.FinishReason and copilotprovider/copilot.go sets update.FinishReason. This mirrors the .NET ChatFinishReason semantics so downstream consumers can reason about why generation stopped regardless of provider.

Tests

• TestMapStopReason: table test asserting each SDK stop_reason constant maps to the expected canonical string (including empty/unknown -> "").
• TestNonStreamingFinishReason: fake-transport run returning stop_reason: tool_use asserts the collected Response.FinishReason == tool_calls.
• TestStreamingFinishReason: streaming run emitting a message_delta with stop_reason: max_tokens asserts Response.FinishReason == length.

go build ./..., go vet ./provider/anthropicprovider/..., and go test ./provider/anthropicprovider/... all pass.

[Copilot]

Pull request overview

This PR brings the Anthropic provider to parity with other providers by populating ResponseUpdate.FinishReason based on Anthropic’s stop_reason for both non-streaming and streaming (via message_delta) flows.

Changes:

• Add mapStopReason(anthropic.StopReason) string helper to translate Anthropic stop_reason values to canonical finish reasons.
• Populate FinishReason on non-streaming responses using resp.StopReason.
• Capture streaming stop_reason from MessageDeltaEvent and emit it on the terminal update, avoiding overwriting with later empty stop reasons.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
provider/anthropicprovider/agent.go	Adds stop-reason mapping and wires FinishReason into non-streaming and streaming terminal updates.
provider/anthropicprovider/agent_test.go	Adds integration-style tests asserting collected FinishReason values for non-streaming and streaming runs.
provider/anthropicprovider/stopreason_internal_test.go	Adds table test for mapStopReason covering known, empty, and unknown reasons.

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[PratikDhanave (PratikDhanave)]

Good catch — fixed in 3f03a5e: added TestStreamingFinishReasonNotClobbered, which streams a max_tokens message_delta followed by a second message_delta with stop_reason:null and asserts FinishReason stays "length", validating the guard against an empty stop_reason overwriting a captured value.

[github-actions]

Parity Review ✅

Scope: Internal implementation fix — mapStopReason is unexported; only ResponseUpdate.FinishReason population behavior changed (was always empty, now populated).

No exported API surface changed. The public-api-change label is not warranted.

Cross-repo alignment confirmed:

• Python (_types.py): FinishReason uses "stop", "length", "tool_calls", "content_filter" — exact same canonical strings mapped here.
• .NET: The PR description cites .NET ChatFinishReason semantics; the ChatClientAgent-based pipeline delegates FinishReason from the underlying Microsoft.Extensions.AI ChatResponse, which uses identical string values.
• Other Go providers (OpenAI, Copilot): already set FinishReason with the same values — this PR brings the Anthropic provider to parity with them.

The mapping table (end_turn/stop_sequence/pause_turn → stop, max_tokens → length, tool_use → tool_calls, refusal → content_filter) is semantically aligned with all three SDKs. No divergence found.

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