Pass through unmapped Anthropic stop reasons

[PratikDhanave (PratikDhanave)]

mapStopReason returned "" for any stop_reason not in its fixed set, so a value the Anthropic API may add later — e.g. model_context_window_exceeded — was reported as no finish reason instead of reaching the caller. This affects both the non-streaming path and the streaming final update.

The Python client passes unmapped reasons through unchanged: FINISH_REASON_MAP.get(stop_reason, stop_reason), with an explicit docstring that newer API values like model_context_window_exceeded must still reach the caller.

Change

• Change the default case to return string(reason). An empty reason still yields "", so the streaming if fr != "" guard is unaffected. StopReason is a plain string in the anthropic SDK.

Test

• Adds a {"model_context_window_exceeded", "model_context_window_exceeded"} row to the shared finishReasonCases table (runs through both TestNonStreamingFinishReason and TestStreamingFinishReason). Fails before (""), passes after.

[Copilot]

Copilot review overview

🟡 Changes recommended

Update the stale function comment to document the new pass-through behavior.

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Review effort: Lite
Findings: 1

Open (1)

• Update comment to document passthrough of unknown reasons · New

What changed in this PR

Updates Anthropic stop-reason handling so unknown non-empty values reach callers unchanged.

Changes:

• Passes through unmapped stop reasons.
• Adds non-streaming and streaming test coverage.

File	Description
provider/​anthropicprovider/​agent.go	Passes through unknown stop reasons.
provider/​anthropicprovider/​agent_test.go	Tests pass-through behavior in both paths.

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[github-actions]

Scope: user-visible behavior

Changed Go contract: No exported symbols changed, but observable runtime behavior does: the FinishReason value returned to callers of the Anthropic provider's Run/streaming APIs for a previously-unmapped stop_reason changes from "" (no finish reason) to the raw reason string (e.g. model_context_window_exceeded). mapStopReason itself remains an unexported helper in provider/anthropicprovider/agent.go.

Upstream evidence reviewed: python/packages/anthropic/agent_framework_anthropic/_chat_client.py — FINISH_REASON_MAP and _map_finish_reason() (lines ~239–258). The Python docstring explicitly states: "Stop reasons that are not in FINISH_REASON_MAP are passed through unchanged so that values added by the Anthropic API, such as model_context_window_exceeded, still reach the caller instead of being reported as no finish reason at all," implemented via FINISH_REASON_MAP.get(stop_reason, stop_reason). No .NET Anthropic-specific stop-reason mapping table was found under dotnet/src/Microsoft.Agents.AI.Anthropic to compare against.

Result: aligned. This change makes the Go mapStopReason default case return string(reason) instead of "", exactly mirroring the Python pass-through behavior (and the same example value, model_context_window_exceeded) cited in the PR description and confirmed above. It's a bug fix that removes a previous Go-only divergence (silently dropping unmapped stop reasons as "") rather than introducing one — Go now matches the documented Python contract. No exported Go API/type/signature changed, so public-api-change is not applicable.

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