feat: add MCP injection support to responses streaming interceptor #143

pawbana · 2026-01-22T17:46:26Z

Adds suppport to MCP tool injection in responses streaming interceptor.

Fixes: #89

pawbana · 2026-01-22T17:46:45Z

feat: add MCP injection support to responses streaming interceptor #143 👈 (View in Graphite)
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mtojek

First review round

intercept/responses/streaming.go

mtojek · 2026-01-22T18:19:40Z

intercept/responses/streaming.go

-		// Set responseID to the first response.id that is set.
-		if responseID == "" && ev.Response.ID != "" {
-			responseID = ev.Response.ID
+	for shouldLoop {


Do we need any security breaker for the maximum number of tools?

I assume something like maximum number of inner loop iterations? I don't think it is necessary.
If I understand MPC correctly there is no such limit when using it in responses API, by default provider will ask for approval but it can be disabled per tool: https://platform.openai.com/docs/guides/tools-connectors-mcp#approvals

So infinite MCP tool call loop in theory could happen when using provider directly with approval disabled.

I think adding configurable max number of inner loop iterations breaker or maybe some more sophisticated solution as a follow up would be nice.

I think adding configurable max number of inner loop iterations breaker or maybe some more sophisticated solution as a follow up would be nice.

SGTM 👍

mtojek · 2026-01-22T18:22:03Z

intercept/responses/streaming.go

+		respCopy = responseCopier{}
+		opts := i.requestOptions(&respCopy)
+		stream := i.newStream(ctx, srv, opts)
+		defer stream.Close()


so this defer will be called after leaving this function, but most likely you want to call it after iteration

Good catch. Added stream.Close() after the iteration (stream can be safely closed multiple times). Though about moving stream processing into separate function but code seemed less clear to me.

Watch out for defer in loops: https://www.jetbrains.com/help/inspectopedia/GoDeferInLoop.html

Good point, although I don't think this loop should iterate much.

This is annoying 😅
I really want to have a defer just in case (very easy to forget in future code changes) but there are too many references to local variables to cleanly move it to separate function.
Would scoping stream processing it into a local func be ok?

intercept/responses/streaming.go

intercept/responses/blocking.go

mtojek · 2026-01-22T18:28:13Z

intercept/responses/blocking.go

+		pending := i.getPendingInjectedToolCalls(response)
+		shouldLoop, err = i.handleInnerAgenticLoop(ctx, pending, response)
 		if err != nil {
 			i.sendCustomErr(ctx, w, http.StatusInternalServerError, err)


Does it mean that if one tool returns an error, we failed the entire prompt?

Depends what "returns an error" means.

if the tool was called and MCP tool returned an error it will be forwarded as a tool call result (see *SingleInjectedToolError fixtures / test cases in ‎responses_integration_test.go)

if there was an error in code executing the MCP tool call eg: empty response or re-marshalling error we fail the prompt.

Thanks to this comment I've found re-marshaling error was ignored. Will add prepareRequestForAgenticLoop error being returned in handleInnerAgenticLoop.

responses_integration_test.go

mtojek · 2026-01-23T10:43:00Z

responses_integration_test.go

+	}
+	t.Cleanup(func() { _ = ln.Close() })
+
+	go func() {


nit: shouldn't we wait for graceful shutdown of this goroutine rather than leaving it on its own?

Added wait to cleanup.

dannykopping

Great work @pawbana!

dannykopping · 2026-01-23T11:31:35Z

intercept/responses/streaming.go

+		respCopy = responseCopier{}
+		opts := i.requestOptions(&respCopy)
+		stream := i.newStream(ctx, srv, opts)
+		defer stream.Close()


Watch out for defer in loops: https://www.jetbrains.com/help/inspectopedia/GoDeferInLoop.html

dannykopping · 2026-01-23T11:34:23Z

intercept/responses/streaming.go

+			// so events can be forwarded as soon as received.
+			// Otherwise loop could iterate so only last response will be forwarded.
+			// This is needed to keep consistency between response.id and response.previous_response_id fields.
+			if i.mcpProxy == nil {


Nit: this doesn't really match the comment, or at least it could be more explicit.
Currently it requires knowledge about how this relates to tool injection.

i.e.

"If no MCP proxy is provided then no tools are injected, ..."

Changed comment.

dannykopping · 2026-01-23T11:34:43Z

intercept/responses/streaming.go

-			err = fmt.Errorf("failed to relay chunk: %w", err)
-			return err
+		streamErr = stream.Err()
+		stream.Close()


This error is not handled.
I've been meaning to add a linter for a while, gonna do it now.

responses_integration_test.go

dannykopping · 2026-01-23T11:45:49Z

intercept/responses/injected_tools.go

+	// Append newly added items to reqPayload field
+	// New items are appended to limit Input re-marshaling.
+	// See responsesInterceptionBase.requestOptions for more details about marshaling issues.
+	for j := originalInputSize; j < len(i.req.Input.OfInputItemList); j++ {


mtojek

Well done 👍

pawbana · 2026-01-23T14:37:09Z

Merge activity

Jan 23, 2:37 PM UTC: A user started a stack merge that includes this pull request via Graphite.
Jan 23, 2:37 PM UTC: @pawbana merged this pull request with Graphite.

pawbana marked this pull request as ready for review January 22, 2026 17:47

pawbana force-pushed the pb/responses-mcp-streaming branch from db0be87 to 2ca985c Compare January 22, 2026 17:49

feat: add MCP injection support to responses streaming interceptor

ba4e96d

pawbana force-pushed the pb/responses-mcp-streaming branch from 2ca985c to ba4e96d Compare January 22, 2026 17:54

pawbana requested review from dannykopping, mtojek and ssncferreira January 22, 2026 17:54

mtojek reviewed Jan 22, 2026

View reviewed changes

review 1

eaec7d1

mtojek reviewed Jan 23, 2026

View reviewed changes

johnstcn mentioned this pull request Jan 23, 2026

fix(coderd): handle rbac.NotAuthorizedError when deleting template coder/coder#21645

Merged

dannykopping approved these changes Jan 23, 2026

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mtojek approved these changes Jan 23, 2026

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pawbana added 3 commits January 23, 2026 13:33

review 2 (stream.Close() func scope)

1455005

review 2: added wait to rejecting listener in test

7006f7a

formatting fix

5c5c528

dannykopping self-requested a review January 23, 2026 14:29

pawbana merged commit b9555f3 into main Jan 23, 2026
2 checks passed

feat: add MCP injection support to responses streaming interceptor #143

feat: add MCP injection support to responses streaming interceptor #143

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pawbana commented Jan 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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pawbana commented Jan 22, 2026 •

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pawbana commented Jan 23, 2026 •

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