fix: trim trailing whitespaces for anthropic and bedrock anthropic provider

[sammaji]

Summary

Trailing whitespace (spaces, newlines, carriage returns, tabs) in the last assistant message is stripped before sending requests to Anthropic and Bedrock (Anthropic models). This prevents API errors that can occur when assistant prefill messages end with whitespace, which Anthropic's API rejects.

Changes

• For the Anthropic provider, all text content blocks in the final assistant message are right-trimmed of whitespace before the request is sent, applied in both chat.go and responses.go.
• For the Bedrock provider, the same trimming is applied to text blocks in the final assistant message, but only when the target model is an Anthropic model (since prefill is an Anthropic-specific concept), applied in both chat.go and responses.go.

Type of change

• Bug fix
• Feature
• Refactor
• Documentation
• Chore/CI

Affected areas

• Core (Go)
• Transports (HTTP)
• Providers/Integrations
• Plugins
• UI (React)
• Docs

How to test

Send a chat completion request where the last message has role: assistant and its content ends with trailing whitespace or newlines.

chat completions

curl --location 'http://localhost:8080/v1/chat/completions' \
--header 'Content-Type: application/json' \
--data '{
    // "model": "anthropic/claude-haiku-4-5-20251001",
    // "model": "anthropic/claude-opus-4-1-20250805",
    // "model": "anthropic/claude-opus-4-20250514",
    // "model": "anthropic/claude-opus-4-5-20251101",
    // "model": "anthropic/claude-opus-4-6",
    // "model": "anthropic/claude-opus-4-7",
    // "model": "anthropic/claude-sonnet-4-20250514",
    // "model": "anthropic/claude-sonnet-4-5-20250929",
    // "model": "anthropic/claude-sonnet-4-6",
    // "model": "bedrock/us.anthropic.claude-3-haiku-20240307-v1:0",
    // "model": "bedrock/us.anthropic.claude-haiku-4-5-20251001-v1:0",
    // "model": "bedrock/us.anthropic.claude-opus-4-1-20250805-v1:0",
    // "model": "bedrock/us.anthropic.claude-opus-4-5-20251101-v1:0",
    // "model": "bedrock/us.anthropic.claude-opus-4-6-v1",
    // "model": "bedrock/us.anthropic.claude-opus-4-7",
    // "model": "bedrock/us.anthropic.claude-sonnet-4-20250514-v1:0",
    // "model": "bedrock/us.anthropic.claude-sonnet-4-5-20250929-v1:0",
    "model": "bedrock/us.anthropic.claude-sonnet-4-6",
    "messages": [
        {
            "role": "user",
            "content": "Hello"
        },
        {
            "role": "assistant",
            "content": "Hello " // trailing whitespace
        }
    ]
}'

anthropic integration (v1/messages)

curl --location 'http://localhost:8080/anthropic/v1/messages' \
--header 'Content-Type: application/json' \
--data '{
    // "model": "anthropic/claude-haiku-4-5-20251001",
    // "model": "anthropic/claude-opus-4-1-20250805",
    // "model": "anthropic/claude-opus-4-20250514",
    // "model": "anthropic/claude-opus-4-5-20251101",
    // "model": "anthropic/claude-opus-4-6",
    // "model": "anthropic/claude-opus-4-7",
    // "model": "anthropic/claude-sonnet-4-20250514",
    // "model": "anthropic/claude-sonnet-4-5-20250929",
    // "model": "anthropic/claude-sonnet-4-6",
    // "model": "bedrock/us.anthropic.claude-3-haiku-20240307-v1:0",
    // "model": "bedrock/us.anthropic.claude-haiku-4-5-20251001-v1:0",
    // "model": "bedrock/us.anthropic.claude-opus-4-1-20250805-v1:0",
    // "model": "bedrock/us.anthropic.claude-opus-4-5-20251101-v1:0",
    // "model": "bedrock/us.anthropic.claude-opus-4-6-v1",
    // "model": "bedrock/us.anthropic.claude-opus-4-7",
    // "model": "bedrock/us.anthropic.claude-sonnet-4-20250514-v1:0",
    // "model": "bedrock/us.anthropic.claude-sonnet-4-5-20250929-v1:0",
    "model": "bedrock/us.anthropic.claude-sonnet-4-6",
    "messages": [
        {
            "role": "user",
            "content": "Hello"
        },
        {
            "role": "assistant",
            "content": "Hey "
    ]
}'

"messages": [
        {
            "role": "user",
            "content": "Hello"
        },
        {
            "role": "assistant",
            "content": [
                {
                    "type": "text",
                    "text": "Hey "
                }
            ]
        }
    ]

Breaking changes

• Yes
• No

Related issues

Security considerations

No security implications. This change only modifies outbound message content formatting before it reaches the provider API.

Checklist

• I read docs/contributing/README.md and followed the guidelines
• I added/updated tests where appropriate
• I updated documentation where needed
• I verified builds succeed (Go and UI)
• I verified the CI pipeline passes locally if applicable

[coderabbitai]

📝 Walkthrough

Walkthrough

Trim trailing right-side whitespace (spaces, newlines, tabs, carriage returns) from the last assistant message's text ContentBlock in Anthropic and Bedrock request/conversion paths across four provider modules.

Changes

Trailing Whitespace Normalization for Assistant Messages

Layer / File(s)	Summary
Anthropic chat request trailing whitespace trimming core/providers/anthropic/chat.go	ToAnthropicChatRequest trims trailing whitespace from the final assistant message's last text ContentBlock before calling stripUnsupportedAnthropicFields.
Anthropic message conversion trailing whitespace trimming core/providers/anthropic/responses.go	ConvertBifrostMessagesToAnthropicMessages (request mode) scans backward through the last assistant message's ContentBlocks and right-trims the first non-nil Text block it finds; a formatting-only brace placement was also adjusted.
Bedrock Anthropic-model trimming in chat request core/providers/bedrock/chat.go	Adds strings import and Anthropic-only post-processing that locates the final assistant message in bedrockReq.Messages, iterates content blocks backward, and applies strings.TrimRight to the first non-nil Text block.
Bedrock Anthropic-model trimming in response conversion core/providers/bedrock/responses.go	ToBedrockResponsesRequest conditionally right-trims trailing whitespace from the last assistant message's content blocks when the target model is an Anthropic model, before continuing request construction.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

I hop through code on nimble paws,
Trimming tails of spaces, line-end gauze.
Four files neat, no trailing fluff—
Messages tidy, snips were not rough.
A rabbit cheers: clean text is enough. 🐇✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: trimming trailing whitespaces for Anthropic and Bedrock Anthropic providers.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description is comprehensive and follows the required template with all essential sections completed and properly filled out.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 05-14-fix_trim_trailing_whitespaces_for_anthropic_and_bedrock_anthropic_provider

---

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[sammaji]

• fix: trim trailing whitespaces for anthropic and bedrock anthropic provider #3496 👈 (View in Graphite)
• dev

This stack of pull requests is managed by Graphite. Learn more about stacking.

[greptile-apps]

Confidence Score: 5/5

Safe to merge — the change is narrowly scoped to outbound request formatting and cannot affect response handling or other providers.

All four modified sites use a backward-iterating loop with an early break, so only the genuinely trailing text block is trimmed; mid-message whitespace is untouched. The Bedrock path is correctly guarded by IsAnthropicModel, the Responses path is guarded by isRequestMessage, and the Bedrock responses path is guarded by bifrostReq.Input != nil. No existing logic is restructured — this is a pure, additive trim step inserted at the end of each conversion function.

No files require special attention.

Important Files Changed

Filename	Overview
core/providers/anthropic/chat.go	Adds targeted whitespace trimming of the last text block in the final assistant message before sending to Anthropic API; correctly uses backward iteration with break to touch only the trailing block.
core/providers/anthropic/responses.go	Same whitespace-trim logic as chat.go, gated on isRequestMessage so response messages are left untouched; also fixes missing newline at end of file.
core/providers/bedrock/chat.go	Adds strings import and applies the same trailing-whitespace trim for Bedrock, correctly scoped to Anthropic models via schemas.IsAnthropicModel.
core/providers/bedrock/responses.go	Mirrors the chat.go fix for the Responses API path; trimming is correctly placed inside the bifrostReq.Input != nil guard where messages are available.

_{Reviews (3): Last reviewed commit: "fix: trim trailing whitespaces for anthr..." | Re-trigger Greptile}

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@core/providers/bedrock/chat.go`:
- Around line 46-57: The final assistant message trimming only trims the first
text block found from the end (the loop breaks), leaving earlier blocks with
trailing whitespace; in the Anthropic-only branch (check using
schemas.IsAnthropicModel and BedrockMessageRoleAssistant) iterate over all
entries in bedrockReq.Messages[lastMsgIndex].Content and for each block with a
non-nil Text field set bedrockReq.Messages[lastMsgIndex].Content[i].Text to
schemas.Ptr(strings.TrimRight(*blocks[i].Text, " \n\r\t")) instead of breaking
after the first trim so every text block in that final assistant message is
trimmed.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 7b4705a7-6f1b-4972-a841-9a2a4b8e3f8d

📥 Commits

Reviewing files that changed from the base of the PR and between d463298 and c531a7e.

📒 Files selected for processing (4)

• core/providers/anthropic/chat.go
• core/providers/anthropic/responses.go
• core/providers/bedrock/chat.go
• core/providers/bedrock/responses.go

🚧 Files skipped from review as they are similar to previous changes (3)

• core/providers/anthropic/chat.go
• core/providers/anthropic/responses.go
• core/providers/bedrock/responses.go

[akshaydeo]

Merge activity

• May 14, 2:32 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• May 14, 2:33 PM UTC: @akshaydeo merged this pull request with Graphite.