gpt-oss-azure-opencode-shim

A small local HTTP shim for Azure-hosted GPT-OSS models. It rewrites the forced tool_choice values that Azure AI Foundry rejects, turns answers that miss the required tool call back into that tool call, reports what it did on every request, and keeps the API key out of the client configuration.

TL;DR

Azure AI Foundry deployments of gpt-oss-120b do not support a forced tool choice:

OpenCode sends tool_choice: "required" for structured output (format: {"type": "json_schema"}), and other clients force a function to get a guaranteed tool call. The shim rewrites every tool_choice other than "auto" or "none" to "auto" before the request reaches Azure.

With "auto", the model sometimes does the work but misses the tool call: it writes the answer as JSON text, or leaks the call's arguments into its reasoning and returns an empty turn. For forced requests, the shim checks the answer against the tool schemas and returns the tool call when exactly one tool matches.

Measured against the real deployment (see Evaluation):

On the 15-scenario synthetic set the polyfill cut stalled turns from 12% to 4% on 2026-09-22, but a rerun on 2026-09-23 found both arms equal within noise (5% and 7%). The structured-output gain is the one that holds.

Every claim in this README is backed by real requests recorded in docs/PROBLEM.md.

The Problem

A client that forces a tool against gpt-oss-120b on Azure AI Foundry gets one of three results:

• Forced function, non-streaming: HTTP 200 with an empty choices array and 2 tokens of usage. The client sees an empty assistant turn, and the tool never runs.
• Forced function, streaming: HTTP 200. The only content is an in-band error event, followed by [DONE]. Clients that skip error events see an empty turn.
• "required": HTTP 400 Request included unsupported tool use. tool_choice 'required' is not supported for the model.

The same request with tool_choice: "auto" returns native tool_calls. The root cause is on the serving side: a forced tool choice needs grammar-constrained decoding, which the deployment's speculative decoding does not support.

When to Use It

Use the shim when a client forces a tool call against this deployment:

• OpenCode structured output (format: {"type": "json_schema"} in the opencode serve API or SDK), which sends "required".
• AI SDK toolChoice: { type: "tool", toolName }, OpenAI SDK tool_choice={"type": "function", ...}, or agents that force a final tool.

You do not need it for plain opencode run. OpenCode 1.18.31 sends tool_choice: "auto" for normal agent turns, and those work against Azure directly with the @ai-sdk/openai-compatible provider.

Limit: "auto" lets the model choose. The polyfill only converts answers that already contain schema-valid JSON. A prose answer ("The files are README.md and …") stays as it is, and the request is counted as failed. See docs/PROBLEM.md.

Quick Start

1. Install

uv tool install gpt-oss-azure-opencode-shim
# or: pipx install gpt-oss-azure-opencode-shim

Both put the gpt-oss-azure-opencode-shim and gpt-oss-azure-opencode-shim-report commands in ~/.local/bin. Add the [otel] extra for OpenTelemetry (uv tool install 'gpt-oss-azure-opencode-shim[otel]'). To try it without installing, run uvx gpt-oss-azure-opencode-shim --help.

2. Configure

The key goes in a file only you can read:

install -m 600 /dev/null ~/.config/gpt-oss-azure-opencode-shim.env
cat >> ~/.config/gpt-oss-azure-opencode-shim.env <<'EOF'
UPSTREAM_URL=https://YOUR_RESOURCE.services.ai.azure.com/openai
AZURE_FOUNDRY_API_KEY=your-key-here
EOF

3. Run as a systemd user service

mkdir -p ~/.config/systemd/user
cat > ~/.config/systemd/user/gpt-oss-azure-opencode-shim.service <<'EOF'
[Unit]
Description=gpt-oss-azure-opencode-shim
After=network.target

[Service]
EnvironmentFile=%h/.config/gpt-oss-azure-opencode-shim.env
ExecStart=%h/.local/bin/gpt-oss-azure-opencode-shim
Restart=always
RestartSec=3

[Install]
WantedBy=default.target
EOF
systemctl --user daemon-reload
systemctl --user enable --now gpt-oss-azure-opencode-shim
curl -s http://127.0.0.1:9526/healthz

To upgrade, run uv tool upgrade gpt-oss-azure-opencode-shim (or pipx upgrade gpt-oss-azure-opencode-shim) and restart the service. From a clone, ./install.sh does steps 1 to 3 with a local venv instead.

4. Point OpenCode at the shim

Add to ~/.config/opencode/opencode.json (or merge with existing — see examples/opencode.json):

{
  "provider": {
    "azure-gpt-oss": {
      "npm": "@ai-sdk/openai-compatible",
      "name": "Azure GPT-OSS via shim",
      "options": {
        "baseURL": "http://127.0.0.1:9526/v1",
        "apiKey": "shim"
      },
      "models": {
        "gpt-oss-120b": {
          "name": "GPT-OSS 120B",
          "reasoning": false,
          "limit": { "context": 131072, "output": 32768 }
        }
      }
    }
  }
}

The apiKey value is a placeholder. The shim replaces it with the real key.

Verify the Fix

examples/curl-tests.sh reproduces each Azure response and checks the shim against the real deployment:

export UPSTREAM_URL=https://YOUR_RESOURCE.services.ai.azure.com/openai
export AZURE_FOUNDRY_API_KEY=your-key-here
./examples/curl-tests.sh

Expected output:

=== 1. Direct Azure — forced function tool_choice (expected: HTTP 200, choices=[]) ===
[PASS] reproduced: HTTP 200 with choices=[]

=== 2. Direct Azure — tool_choice="required" (expected: HTTP 400 UnsupportedToolUse) ===
[PASS] reproduced: HTTP 400 UnsupportedToolUse

=== 3. Direct Azure — tool_choice="auto" (expected: tool_calls) ===
[PASS] Azure emits native tool_calls with tool_choice=auto

=== 4. Via shim — forced function and "required" are rewritten to auto ===
[PASS] tool_choice={"type":"function","function":{"name":"get_files"}} -> native tool_calls
[PASS] tool_choice="required" -> native tool_calls

All checks passed.

The script exits with a non-zero status if any check fails, including authentication errors.

How It Works

┌──────────┐         ┌────────────────┐         ┌──────────────────┐
│  client  │ ──────▶ │      shim      │ ──────▶ │  Azure Foundry   │
│ OpenCode │         │ 127.0.0.1:9526 │         │  gpt-oss-120b    │
└──────────┘         └────────────────┘         └──────────────────┘
                            │
                            ├── reject browser and non-local requests
                            ├── rewrite forced tool_choice → "auto"
                            ├── inject the API key
                            ├── forced requests: check answer, repair tool call
                            ├── relay response, headers, and SSE stream
                            └── x-shim-outcome header + /metrics

1. Tool choice rewriting

For POST .../chat/completions, any tool_choice other than "auto" or "none" becomes "auto". Each rewrite is logged:

INFO gpt_oss_shim: sanitized request: tool_choice='required' -> 'auto'

2. Tool-call polyfill

For a forced request, the shim buffers the answer (streamed or not) and inspects it:

• The model called a tool: the answer passes unchanged (native).
• The answer text is a JSON object, with or without a Markdown fence: it becomes a tool call.
• The answer text is empty and the reasoning ends with a JSON object: it becomes a tool call. This is how gpt-oss leaks a call it did not emit.
• Anything else passes unchanged (failed).

Matching is strict. The JSON must validate against the tool's JSON Schema and use only declared top-level properties, and exactly one candidate tool may match (only the named tool for a forced function). The shim never guesses. For a rescued stream, the reasoning and usage events are kept, and the answer text is replaced by one tool-call chunk with finish_reason: "tool_calls".

Only forced requests are buffered, so other requests keep streaming token by token.

SHIM_TOOL_POLYFILL selects the mode:

3. Outcomes and metrics

Every forwarded request gets an x-shim-outcome response header:

GET /metrics exposes the same outcomes in Prometheus format:

shim_requests_total{outcome="native"} 59.0
shim_requests_total{outcome="rescued"} 13.0
shim_requests_total{outcome="failed"} 3.0
shim_forced_request_duration_seconds_sum{outcome="rescued"} 15.63
shim_forced_request_duration_seconds_count{outcome="rescued"} 13.0

shim_forced_request_duration_seconds is the time a forced request waits before its first byte, because the shim buffers it. In observe mode, shim_polyfill_observed_total{outcome} counts what the repair would have done while every answer stays unchanged (x-shim-outcome: rewritten). native_rate = native / forced also shows when Azure starts supporting forced tool choice, which is when the polyfill stops being needed.

4. Credentials

The client sends a placeholder key. The shim sends the real key as both api-key and Authorization: Bearer (Azure accepts either). The key lives only in the shim's environment file, which the Quick Start and install.sh create with mode 600.

5. Header hygiene

The shim sets its own Content-Type and drops the client's copy, plus Authorization, Accept-Encoding, and hop-by-hop headers. Azure rejects a duplicated Content-Type (application/json,application/json) with HTTP 400.

6. Upstream failures

• One shared HTTP client keeps connections to Azure open between requests.
• Connect timeout 10 s, read timeout 600 s between bytes. A stalled upstream returns HTTP 504 instead of hanging.
• Transport errors return HTTP 502 with an OpenAI-style error body.
• If a stream breaks mid-way, the shim ends it with a data: {"error": ...} event, so the client reports an error instead of a silent, truncated answer.
• Azure response headers such as retry-after, x-ratelimit-*, and x-request-id reach the client.
• Logs never contain the Azure resource name: the HTTP client's request lines stay hidden unless SHIM_LOG_LEVEL=debug.

Tracing and fixtures

OpenTelemetry (optional)

uv tool install 'gpt-oss-azure-opencode-shim[otel]'   # or: pipx install '...[otel]'
OTEL_EXPORTER_OTLP_ENDPOINT=http://127.0.0.1:4318 systemctl --user restart gpt-oss-azure-opencode-shim

Each chat request becomes one span named chat <model>, with GenAI semantic conventions (gen_ai.request.model, gen_ai.response.model, gen_ai.usage.input_tokens, gen_ai.usage.output_tokens, gen_ai.response.finish_reasons) plus shim.outcome, shim.polyfill.mode, shim.tool_choice.forced and, in observe mode, shim.observed_outcome. A streamed request's span ends with its stream.

The gen_ai.* attributes describe the answer as the model produced it, so a rescued answer keeps finish_reason: "stop" while shim.outcome is rescued. The upstream host is never recorded, because it contains the Azure resource name. OTEL_TRACES_EXPORTER=console prints spans to the log instead of sending them.

Traces to fixtures

Set SHIM_TRACE_DIR and the shim records the forced requests worth replaying (by default rescued, failed and empty_choices, configurable with SHIM_TRACE_OUTCOMES):

SHIM_TRACE_DIR=~/.local/share/gpt-oss-azure-opencode-shim/traces

A trace holds the candidate tools, the original tool_choice and the raw upstream answer. It never holds messages, so prompts, file contents and tool results stay out; the directory is created 0700 and the files 0600. The recorded answer is still model output, so review a trace before committing it.

Traces go to one file per day and are bounded. Each record repeats the tool definitions (36 requests produced 478 KB in testing), so the shim deletes days older than SHIM_TRACE_RETENTION_DAYS (default 14) and deletes the oldest days of traces first when the directory passes SHIM_TRACE_MAX_MB (default 100); outcome logs leave only through retention. When today's traces alone reach the cap, new traces are dropped until the next day and counted in shim_traces_dropped_total. Only files named traces-YYYY-MM-DD.jsonl or outcomes-YYYY-MM-DD.jsonl are ever deleted.

Production outcomes

With SHIM_TRACE_DIR set, the shim also appends one small line per chat request to outcomes-YYYY-MM-DD.jsonl: outcome, polyfill mode, whether the client forced a tool call, HTTP status, latency and model name. It holds no content and is kept even when traces hit the cap. Prometheus counters reset on every restart, while this log gives rates over days:

gpt-oss-azure-opencode-shim-report ~/.local/share/gpt-oss-azure-opencode-shim/traces --days 7

The report shows the share of forced requests and the native, rescued, failed and stalled rates with 95% Wilson intervals. It counts only forced requests that got HTTP 200; upstream errors are listed apart.

Promote one into a regression fixture:

python -m evals.fixtures list ~/.local/share/gpt-oss-azure-opencode-shim/traces/traces-2026-09-22.jsonl
python -m evals.fixtures promote TRACES.jsonl --line 41 --id reasoning-leak \
  --description "Empty content; the call's arguments end the reasoning" \
  --keep-tools glob,read,StructuredOutput --replace /home/me/project=/repo --coalesce

--keep-tools drops tools that reveal local setup and --replace scrubs strings. Streamed text arrives split across events, so a replacement that still matches the joined text is refused with a pointer to --coalesce, which merges the text deltas first. tests/test_fixture_replay.py then replays every fixture offline on each CI run: a change in the polyfill that alters a decision on a real answer fails there.

The fixtures in tests/fixtures/polyfill/ were recorded this way from gpt-oss-120b and cover both rescued shapes, the three shapes left unchanged (including a leaked call whose JSON is truncated, where completing it would mean inventing arguments) and two native tool calls.

Evaluation

evals/tool_choice_eval.py sends 15 forced-tool scenarios (evals/scenarios.py) to each target: final-answer steps after a tool result, single-turn extraction into a schema, forced functions, and "required" with action tools (7 of them streamed). A run succeeds when the first tool call names the expected tool and its arguments validate against that tool's schema. A turn is stalled when the answer has no tool call at all.

Results on 2026-09-22, gpt-oss-120b, 15 scenarios × 4 repetitions per target:

What the numbers show:

• Strict success is the same within noise on this synthetic set. The 4 rescued turns were calls to an intermediate tool (read, bash) that the model had leaked into its reasoning, while the scenario expected the final StructuredOutput. In an agent loop that is progress, not a stall, but the strict scorer counts it as a miss.
• The polyfill cuts stalled turns from 12% to 4%.
• Azure returned HTTP 500 to 6 requests spread over all three targets. Those are upstream errors, not shim failures.

End-to-end with OpenCode 1.18.31 structured output (format: json_schema, 10 runs each, same session):

OpenCode's structured-output prompt makes the model write the answer as JSON, which the polyfill can convert. That is why the gain is large here and small on the synthetic set.

Rerun on 2026-09-23 with v0.3.1, which also scores progress (the first tool call is any offered tool with schema-valid arguments, as an agent loop needs) and prints 95% Wilson intervals. Both arms ran from the same checkout in the same session; the rewrite-only arm is the polyfill in observe mode, which returns the upstream answer unchanged:

On this day the two arms are the same within noise: the intervals overlap on every metric, and the polyfill rescued 2 turns out of 60. The rewrite-only arm stalled on 5% of turns against 12% the day before, with the same scenarios and shim logic, so the upstream model's behavior varies from day to day by more than the polyfill changes it on this set. The synthetic set does not show a polyfill gain; the OpenCode structured-output result above does. Production rates will come from the outcome log (see Production outcomes).

Run it yourself (costs a few cents of tokens):

python -m evals.tool_choice_eval \
  --target direct=$UPSTREAM_URL/v1/chat/completions \
  --target shim=http://127.0.0.1:9526/v1/chat/completions \
  --repeat 4 --out eval-results.json

Security

The shim adds a real API key to every request it forwards and has no inbound authentication. It is built for a single local user:

• It binds to 127.0.0.1 by default and logs a warning if SHIM_HOST is not a loopback address.
• It returns HTTP 403 for requests with an Origin header or a Sec-Fetch-Site value other than none. Web pages cannot use the shim through cross-site requests.
• It returns HTTP 403 if the Host header is not localhost, 127.0.0.1, ::1, or a name in SHIM_ALLOWED_HOSTS. This blocks DNS rebinding.

Do not expose the shim on a network interface. The Host check does not stop a client on the network that sends Host: localhost.

Configuration Reference

Compatibility

Likely applies to gpt-oss-20b as well (untested).

Not a protocol translator. The shim assumes both sides speak OpenAI Chat Completions. Use the @ai-sdk/openai-compatible provider in OpenCode. The @ai-sdk/openai package failed against the same deployment with Invalid parameter: the model does not support one or more of the provided input parameters, which this shim does not address.

Does not fix OpenCode's empty-assistant-materialization bug when the model returns only reasoning. That's an upstream issue.

Development

git clone https://github.com/Gabrielm3/gpt-oss-azure-opencode-shim.git
cd gpt-oss-azure-opencode-shim
python3 -m venv .venv
./.venv/bin/pip install -e ".[dev]"
./.venv/bin/pytest -v
./.venv/bin/ruff check src tests evals
./.venv/bin/ruff format --check src tests evals

Releasing

Releases go to PyPI through trusted publishing: no API token is stored anywhere, and every file carries a PEP 740 attestation.

1. Bump version in pyproject.toml (the only place it lives) and merge.
2. Tag and push: git tag -a v1.2.3 -m v1.2.3 && git push origin v1.2.3.
3. release.yml builds once, checks the tag against the version, runs scripts/check-dist.sh (metadata, wheel in a clean venv, full tests from the sdist), then waits for approval on the pypi environment before publishing and attaching the files to the GitHub release.

Running the workflow by hand (gh workflow run release.yml) is a dry run to TestPyPI. Actions are pinned by commit SHA and build tools by hash, and Dependabot keeps both current.

Contributing

Issues and PRs are welcome. Run pytest, ruff check src tests evals, and ruff format --check src tests evals before opening a PR.

License

MIT — see LICENSE.