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+# Native Responses Namespace Compatibility
+
+## Context
+
+Codex clients can send private `namespace` tool declarations and tag historical
+`function_call` input items with a `namespace`. The chat-completions fallback
+bridge already flattens these declarations into ordinary function tools and
+restores the namespace on responses. The native Responses OAuth path currently
+forwards the private fields unchanged. Upstreams that implement only the public
+Responses schema reject them, and sub2api surfaces the failure as a 502.
+
+## Scope
+
+Extend namespace compatibility to the native Responses OAuth path without
+changing the existing chat-completions bridge or ordinary function tools.
+
+The request transformation must:
+
+- flatten every function child of a `type: "namespace"` tool into a top-level
+  function tool using the existing collision-safe naming convention;
+- rewrite namespace-qualified historical `function_call` items to the same
+  flattened name and remove their private `namespace` field;
+- rewrite namespace-qualified `tool_choice` values when present;
+- reject ambiguous flattened names with a clear client error instead of
+  forwarding a request that cannot be routed safely;
+- preserve all unrelated request fields and tools.
+
+The response transformation must:
+
+- restore a flattened tool call to its original child name plus `namespace`;
+- cover both JSON responses and every relevant SSE lifecycle event, including
+  the final completed response;
+- leave tool calls that are not in the request mapping unchanged.
+
+## Design
+
+Extract the namespace flattening and mapping behavior that is currently tied to
+the chat-completions bridge into reusable helpers in `internal/pkg/apicompat`.
+The helpers will operate on Responses request structures and expose the mapping
+from flattened names to original namespace/name pairs.
+
+The native OAuth forwarding path will apply the request helper during its
+existing Codex OAuth normalization pass. The resulting mapping is request-local
+and will be passed to the native response handling code. JSON and SSE response
+rewriters will use it to restore function-call identity before bytes are sent to
+the client. No mapping is persisted across requests because the request contains
+the declarations needed to reconstruct it.
+
+This change targets the native HTTP Responses forwarding path demonstrated by
+the issue. WebSocket forwarding is outside the scope unless a failing regression
+test proves that it shares the same HTTP transformation boundary.
+
+## Error Handling
+
+Malformed namespace declarations that cannot be transformed safely, including
+flattened-name collisions, return a 400-class invalid-request response. Upstream
+transport failures continue through the existing error and failover handling.
+Response rewriting is conservative: unrecognized event shapes pass through
+unchanged, while valid mapped function-call objects are restored.
+
+## Testing
+
+Tests will be written before implementation and will demonstrate the regression
+against the current code. Coverage includes:
+
+- OAuth native request forwarding with namespace tools and historical calls;
+- namespace `tool_choice` rewriting;
+- collision rejection;
+- non-stream JSON response restoration;
+- streaming SSE restoration across added, done, and completed events;
+- ordinary function tools and payload fields remaining unchanged;
+- targeted service and `apicompat` test suites, followed by formatting and the
+  repository's relevant backend verification commands.
+
+## Non-goals
+
+- Changing the Codex client or requiring a client downgrade.
+- Forcing OAuth accounts through the chat-completions fallback bridge.
+- Changing connector security policy from issue #3408.
+- Adding persistent namespace state or changing account scheduling behavior.