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fix(ai): send OpenAI agent turns through the Responses API - #1816

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fix/openai-responses-tool-turns

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@chhoumann chhoumann commented Sep 26, 2026 •

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Summary

Fixes two tool-calling correctness issues.

1. OpenAI agent turns now use the Responses API. gpt-6-* and gpt-5.6-* reason by default, and /v1/chat/completions rejects function tools for them: "Function tools with reasoning_effort are not supported for gpt-6-luna in /v1/chat/completions. To use function tools, use /v1/responses or set reasoning_effort to 'none'." #1811 retried every tool turn with reasoning_effort: "none". That doubled the requests (one 400 per turn) and ran tool loops with reasoning off.

  • getChatWire(provider) sends OpenAI-kind providers on api.openai.com to a new openai-responses wire (/v1/responses). Every other OpenAI-compatible endpoint keeps Chat Completions, including lookalike hosts such as api.openai.com.evil.example.
  • Requests are stateless (store: false, the same no-retention default as Chat Completions) and ask for reasoning.encrypted_content. Each turn's output items go back verbatim through providerRaw, the mechanism Gemini's thought signatures already use, so reasoning carries across tool calls.
  • modelOptions keep their Chat Completions names: reasoning_effort becomes reasoning.effort, and max_tokens/max_completion_tokens become max_output_tokens. Everything else passes through.
  • toolReasoningRetryBody and its tests are removed.
  • A safety refusal (a refusal content part) comes back as the answer text with stop reason refusal, not as an empty string (from review).

2. Anthropic forced tool choice. Anthropic's docs (Forcing tool use, errors) say Claude Opus 5.5, Fable 5.1 and Mythos 5.1 return 400 invalid_request_error: tool_choice: type "tool" and "any" are not supported for this model.

  • No model metadata carries this. The Models API capabilities object has no tool-choice field, and models.dev has none either. So the documented error text is the signal, not a list of model ids, which also covers models that adopt the rule later.
  • QuickAdd adds a clear explanation to that error: "claude-opus-5-5 can't be forced to call a tool, so toolChoice "required" and named tools don't work with it. Use toolChoice "auto" (the default) and say in the prompt when to call the tool, or pass a schema to get a fixed JSON shape."
  • There is deliberately no silent fallback to auto. That would drop the script author's "must call a tool" guarantee without telling them.

src/gui/AIAssistantProvidersModal.ts is untouched.

Proof

Real Obsidian (1.13.7, throwaway e2e vault), quickAddApi.ai.agent two-step tool loop

Each outgoing requestUrl was recorded at the Electron IPC boundary (path plus a body summary, no headers). Prompt: add 17 + 25 with the tool, then add 100 with the tool.

Before (origin/master @ 7ee3e57): 6 requests per run. Every turn is a 400 followed by a retry with reasoning off.

## gpt-6-luna {"text": "142", "finishReason": "stop", "toolCalls": [[17, 25], [42, 100]]}
    {"path": "/v1/chat/completions", "tools": ["add"], "reasoning_effort": null, ...}
    {"path": "/v1/chat/completions", "tools": ["add"], "reasoning_effort": "none", ...}
    {"path": "/v1/chat/completions", "tools": ["add"], "reasoning_effort": null, ...}
    {"path": "/v1/chat/completions", "tools": ["add"], "reasoning_effort": "none", ...}
    {"path": "/v1/chat/completions", "tools": ["add"], "reasoning_effort": null, ...}
    {"path": "/v1/chat/completions", "tools": ["add"], "reasoning_effort": "none", ...}
(gpt-5.6-terra: identical pattern)

After (this branch): 3 requests per run, reasoning on. Reasoning items are echoed across turns.

## gpt-6-luna {"text": "142", "finishReason": "stop", "toolCalls": [[17, 25], [42, 100]]}
    {"path": "/v1/responses", "tools": ["add"], "reasoning_effort": null, "inputItemTypes": ["system", "user"]}
    {"path": "/v1/responses", "tools": ["add"], "reasoning_effort": null, "inputItemTypes": ["system", "user", "function_call", "function_call_output"]}
    {"path": "/v1/responses", "tools": ["add"], "reasoning_effort": null, "inputItemTypes": ["system", "user", "function_call", "function_call_output", "function_call", "function_call_output"]}
## gpt-5.6-terra {"text": "142", "finishReason": "stop", "toolCalls": [[17, 25], [42, 100]]}
    {"path": "/v1/responses", "tools": ["add"], "reasoning_effort": null, "inputItemTypes": ["system", "user"]}
    {"path": "/v1/responses", "tools": ["add"], "reasoning_effort": null, "inputItemTypes": ["system", "user", "reasoning", "function_call", "function_call_output"]}
    {"path": "/v1/responses", "tools": ["add"], "reasoning_effort": null, "inputItemTypes": ["system", "user", "reasoning", "function_call", "function_call_output", "function_call", "function_call_output"]}
## structured (ai.agent with schema, gpt-6-luna) {"object": {"title": "Hello World", "tags": ["alpha", "beta"]}}
    {"path": "/v1/responses", "tools": [], "format": "json_schema"}

dev:errors: No errors captured.

Anthropic and OpenAI-compatible routing in real Obsidian. No Anthropic key was available, so a local mock returned Anthropic's documented 400 verbatim whenever tool_choice was any/tool. The agent ran against claude-opus-5-5:

"required"                       -> error: ...tool_choice: type "tool" and "any" are not supported for this model. claude-opus-5-5 can't be forced to call a tool, so toolChoice "required" and named tools don't work with it. Use toolChoice "auto" (the default) ...
{ type: "tool", toolName: "add" } -> same error
"auto"                           -> text "auto ok"
OpenAI-compatible provider (non-openai.com endpoint) -> POST /v1/chat/completions, text "compat ok"

Live wire e2e (src/ai/tools/openai.e2e.test.ts, now covering both wires)

OPENAI_E2E_MODEL=gpt-6-luna     Tests  4 passed (4)   # Responses + Chat Completions (gpt-5-mini) suites
OPENAI_E2E_MODEL=gpt-5.6-luna   Tests  4 passed (4)
OPENAI_E2E_MODEL=gpt-6-sol      Responses suite: 2 passed
OPENAI_E2E_MODEL=gpt-6-astra    Responses suite: 2 passed
OPENAI_E2E_MODEL=gpt-5.6-sol    Responses suite: 2 passed
OPENAI_E2E_MODEL=gpt-5.6-terra  Responses suite: 2 passed
OPENAI_E2E_MODEL=gpt-4o-mini    Responses suite: 2 passed
Baseline, Chat Completions wire with gpt-6-luna: 1 failed
  Error: OpenAI 400: "Function tools with reasoning_effort are not supported for gpt-6-luna in /v1/chat/completions. ..."

Probes run while designing this: a verbatim echo of reasoning (encrypted_content) plus function_call items works with store: false on gpt-5.6-sol and gpt-6-luna. frequency_penalty/presence_penalty are accepted on Responses by gpt-4o-mini/gpt-4.1-mini, and include: ["reasoning.encrypted_content"] is accepted by non-reasoning models.

Review follow-ups (checked live)

  • Legacy chat models on /v1/responses: gpt-3.5-turbo, gpt-4, gpt-4-turbo, gpt-4o all return completed with a function_call, so routing by host is safe.
  • Penalties on Responses: supported. They are accepted and echoed back, while unknown fields are rejected ("Unknown parameter: 'bogus_param'.").
  • Re-ran the Obsidian agent check and the gpt-6-luna live e2e on 5df9e3a. Same results as above, 4/4 passing.

Regression tests

  • src/ai/OpenAIRequest.toolTurns.test.ts goes through chatRequest. It covers the /v1/responses routing, call_id rather than the item id, verbatim echo with function_call_output, no reasoning retry, Chat Completions for third-party/proxy/lookalike endpoints, and the Anthropic forced-choice explanation (with other tool_choice errors untouched). The three behaviour tests fail on origin/master.
  • providerToolMapping.test.ts: Responses body shape, modelOptions renames and precedence, strict tools, named tool_choice, text.format, rebuilt function_call items, the max_output_tokens → length stop reason, and unparseable arguments.
  • Provider.test.ts (getChatWire) and providerErrors.test.ts (isForcedToolChoiceUnsupportedError).
  • pnpm run test: 450 files, 5849 passed. pnpm run build-with-lint: clean.

Release / migration impact

gpt-6-* and gpt-5.6-* reason by default, and /v1/chat/completions rejects
function tools for them unless reasoning_effort is "none". #1811 worked
around that by retrying every tool turn with reasoning off, which cost a
failed request per turn and ran tool loops without reasoning.

Agent turns to OpenAI's own endpoint (api.openai.com) now use /v1/responses.
Requests are stateless (store: false) and include encrypted reasoning, and
each turn's output items are echoed back verbatim, so reasoning carries
across tool calls. modelOptions keep their Chat Completions names:
reasoning_effort and max_tokens are mapped to reasoning.effort and
max_output_tokens. OpenAI-compatible third-party endpoints keep Chat
Completions, and the reasoning_effort retry is removed.

Claude Opus 5.5 and Fable 5.1 return a documented 400 for tool_choice
any/tool. Neither Anthropic's Models API capabilities nor models.dev
expose that as metadata, so QuickAdd recognizes the documented error and
explains the fix (use "auto" with a prompt hint, or a schema) instead of
silently weakening a forced choice to "auto".

Amp-Thread-ID: https://ampcode.com/threads/T-01a0df9e-e548-71c1-acd1-5cee32990534
Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: Christian Bager Bach Houmann <christian@bagerbach.com>
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📝 Walkthrough

Walkthrough

Chat requests to the exact OpenAI API hostname now use the Responses API. The change adds Responses request and response mapping, preserves tool-turn output items, removes reasoning-effort retries for rejected tool turns, and adds guidance for a specific Anthropic forced-tool-choice error.

Changes

OpenAI chat request flow

Layer / File(s) Summary
Select and route the chat wire
src/ai/Provider.ts, src/ai/Provider.test.ts, src/ai/providerRequest.ts, src/ai/tools/providerToolMapping.ts, src/ai/OpenAIRequest.ts
Adds ChatWire selection based on provider kind and endpoint hostname. Requests selected for OpenAI Responses route to /responses; other providers retain their existing wire formats.
Map Responses requests and results
src/ai/tools/providerToolMapping.ts, src/ai/tools/providerToolMapping.test.ts, src/ai/tools/openai.e2e.test.ts
Builds Responses input, tool, and structured-output fields, and parses output, usage, stop status, and raw items. Tests cover mapping and live Responses and Chat Completions paths.
Handle tool turns and provider errors
src/ai/OpenAIRequest.ts, src/ai/providerErrors.ts, src/ai/providerErrors.test.ts, src/ai/OpenAIRequest.toolTurns.test.ts, src/ai/OpenAIRequest.toolReasoning.test.ts, src/ai/OpenAIRequest.sampling.test.ts, docs/src/content/docs/docs/QuickAddAPI.md
Removes reasoning-effort retries for rejected tool turns and adds guidance for Anthropic’s unsupported forced-tool-choice error. Updates tool-turn tests and API documentation; the sampling retry test now uses a Responses response.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant OpenAIRequest
  participant providerToolMapping
  participant providerRequest
  participant OpenAIResponsesAPI
  OpenAIRequest->>providerToolMapping: buildChatBody for openai-responses
  OpenAIRequest->>providerRequest: dispatchProviderRequest
  providerRequest->>OpenAIResponsesAPI: POST /responses
  OpenAIResponsesAPI-->>OpenAIRequest: response JSON
  OpenAIRequest->>providerToolMapping: parseChatResponse
Loading

Merge Risk: ⚪ Minimal · up to 5df9e

The reviewed change has no newly established issue requiring a fix before merge. Normal build and test checks still apply.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 5df9e

The new conversation flow warrants review because it carries tool calls across requests. The inspected path retains the existing checks and approvals before a tool runs, and no new access to local tools was established. Broader security coverage remains incomplete.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

  • inferred — The changed wire affects agent conversations using OpenAI-kind providers at api.openai.com, but does not itself expand the set of locally registered tools or route lookalike hosts to the new endpoint.

Trust Boundaries and Controls

  • observed — Provider-produced function names and arguments enter the existing tool boundary as untrusted calls. Unknown tools and invalid arguments return errors; registered tools require confirmation before execution.

Resilience and Maintainability Implications

  • inferred — Retries of a provider request cannot by themselves repeat a local tool execution: parsing and handoff to the tool loop occur only after the retrying request completes.
🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

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Docstring Coverage ⚠️ Warning Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 21 functions across 11 files. Write docstrings for the functions missing them to satisfy the coverage threshold.
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Out of Scope Changes check ✅ Passed Check skipped because no linked issues were found for this pull request.
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check ✅ Passed The title clearly and concisely describes the primary change: routing OpenAI agent turns through the Responses API.
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A rabbit checks the message flow,
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In @src/ai/tools/providerToolMapping.ts:
- Around line 291-299: Update buildOpenAIResponsesBody to remove
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Comment thread src/ai/tools/providerToolMapping.ts
A safety refusal arrives as a refusal content part, not output_text, so the
agent returned an empty string and reported a normal stop. Return the
refusal's explanation and mark the stop reason as refusal.

Amp-Thread-ID: https://ampcode.com/threads/T-01a0df9e-e548-71c1-acd1-5cee32990534
Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: Christian Bager Bach Houmann <christian@bagerbach.com>
@chhoumann
chhoumann merged commit 9e43828 into master Sep 26, 2026
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Follow-up: live Anthropic verification (after merge, on master @ 9e43828)

Christian provided an Anthropic key, so the forced-tool-choice path is now checked against the real API. Before, it was checked only against a mock.

Raw API, 2026-09-26. The live error text matches the documented text, and the pattern in providerErrors.ts:

claude-opus-5-5  any / tool  -> 400 invalid_request_error: tool_choice: type "tool" and "any" are not supported for this model.
claude-opus-5-5  auto        -> tool_use
claude-fable-5-1 any / tool  -> 400 (same message)
claude-fable-5-1 auto        -> tool_use
claude-opus-5    any / tool / auto -> tool_use (forced choice still works on older models)

Live wire e2e (anthropic.e2e.test.ts, tool loop + structured output): ANTHROPIC_E2E_MODEL = claude-opus-5-5, claude-fable-5-1, claude-opus-5, claude-sonnet-5 → Tests 2 passed (2) each.

Real Obsidian (1.13.7, throwaway vault), quickAddApi.ai.agent, real api.anthropic.com:

claude-opus-5-5  toolChoice "required"                  -> 1 request, error: ...not supported for this model. claude-opus-5-5 can't be forced to call a tool, so toolChoice "required" and named tools don't work with it. Use toolChoice "auto" (the default) ...
claude-opus-5-5  { type: "tool", toolName: "add" }      -> same error
claude-opus-5-5  "auto"                                 -> text "142", tool calls [17,25], [42,100], 3 requests
claude-fable-5-1 (same three cases)                     -> same results

dev:errors: No errors captured.

chhoumann added a commit that referenced this pull request Sep 26, 2026
…rns on gateways (#1819)

* fix(ai): return Chat Completions refusals and retry reasoning tool turns on gateways

Chat Completions refusals leave content null and put the explanation in
message.refusal, so the agent returned an empty string as a normal stop,
and a schema call spent a repair request on it. Return the refusal text
with stop reason "refusal", and skip the structured-output repair after a
refusal on either OpenAI wire.

#1816 moved api.openai.com tool turns to the Responses API and dropped
the reasoning_effort "none" retry. Gateways (Azure OpenAI, OpenRouter,
LiteLLM) can still serve gpt-6 and gpt-5.6 over Chat Completions, where
function tools are rejected while the model reasons. Restore the retry for
the Chat Completions wire only; api.openai.com never takes it.

Amp-Thread-ID: https://ampcode.com/threads/T-01a0df9e-e548-71c1-acd1-5cee32990534
Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: Christian Bager Bach Houmann <christian@bagerbach.com>

* fix(ai): never return a refusal as the structured object

Check for a refusal before parsing, so refusal text that happens to be
valid JSON can't become result.object.

Amp-Thread-ID: https://ampcode.com/threads/T-01a0df9e-e548-71c1-acd1-5cee32990534
Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: Christian Bager Bach Houmann <christian@bagerbach.com>

---------

Co-authored-by: Amp <amp@ampcode.com>
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