feat(contrib/mcp-go): Initial `mcp-go` tracer implementation #4100

jboolean · 2025-11-05T18:37:17Z

What does this PR do?

Adds contrib library for mcp-go. Uses it to record LLMObs Tool spans for tool calls.

This is the base of a Graphite stack.

Motivation

Tech spec for this project

Closes MLOB-4372

Reviewer's Checklist

Changed code has unit tests for its functionality at or near 100% coverage.
System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
There is a benchmark for any new code, or changes to existing code.
If this interacts with the agent in a new way, a system test has been added.
New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
Add an appropriate team label so this PR gets put in the right place for the release notes.
Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Testing

In addition to automated tests, this was tested locally with Datadog MCP Server

Unsure? Have a question? Request a review!

jboolean · 2025-11-05T18:37:37Z

feat(contrib/mcp-go): Trace MCP session initializations with MLObs spans #4101 : 3 dependent PRs (#4106 , #4115 , #4122 )
feat(contrib/mcp-go): Initial mcp-go tracer implementation #4100 👈 (View in Graphite)
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pr-commenter · 2025-11-05T18:42:29Z

Benchmarks

Benchmark execution time: 2025-11-13 19:07:27

Comparing candidate commit 38e2a00 in PR branch jb/contrib-mcp-go-base with baseline commit 2b60c19 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 6 metrics, 0 unstable metrics.

datadog-official · 2025-11-05T18:45:52Z

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.

🔗 Commit SHA: 38e2a00 | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

jboolean · 2025-11-05T18:46:19Z

contrib/mark3labs/mcp-go/hooks.go

+// Hooks provides Datadog tracing for MCP servers.
+type Hooks struct {
+	// Stores data between start and end of tool calls. In-memory works because it must be on the same instance.
+	toolCache *ttlcache.Cache[any, *llmobs.ToolSpan]


The in-memory cache just stores data between the start and end hooks. Creating a middleware would have avoided this, but would have been harder to automatically add to all tools. We'll also use hooks for session initialization spans (another PR on this stack).

Unfortunately this cache implementation does need to be shut down manually.

I experimented with but decided against cache customization options (max size, ttl).

Another downside I see from this hooks option is that it does not allow to pass down the newly created context with the span to the handler, so if the user starts spans in the handler they won't be children of the spans we are creating here.

I agree, especially since users may want to add tags to the existing span in the handler. Just know that there will be asymmetry/inconsistency between tools and initialization because I don't see any initialization middleware.

rarguelloF

Mostly looking good! just a few nits / comments

contrib/mark3labs/mcp-go/README.md

contrib/mark3labs/mcp-go/example_test.go

rarguelloF · 2025-11-07T14:09:07Z

contrib/mark3labs/mcp-go/hooks.go

+
+// onBeforeCallTool is called before a tool is executed.
+func (h *Hooks) onBeforeCallTool(ctx context.Context, id any, request *mcp.CallToolRequest) {
+	toolSpan, _ := llmobs.StartToolSpan(ctx, request.Params.Name)


I'm not sure if other tracers are providing any kind of info about these spans coming from the contrib package (in APM we would set tag component: "mark3labs/mcp-go").

Since this call also creates an APM span, we should do it at least for the APM part (we might need to change the llmobs package a little bit or add a new option).

@sabrenner says that's just for APM spans. But LLMObs does set an integration tag so I'll add that.

Actually, @rarguelloF I see integration as a private field on the Span struct but no actual way to set it. I'll add that to llmobs.

contrib/mark3labs/mcp-go/hooks_test.go

instrumentation/packages.go

internal/env/supported_configurations.json

rarguelloF

Mostly looking good! just a few nits / comments

genesor

LGTM, two nits

instrumentation/packages.go

contrib/mark3labs/mcp-go/mcpgo.go

contrib/mark3labs/mcp-go/mcpgo_test.go

internal/env/supported_configurations.json

hannahkm · 2025-11-12T22:26:53Z

internal/env/supported_configurations.gen.go

 	"DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED":                        {},
 	"DD_TRACE_ABANDONED_SPAN_TIMEOUT":                                 {},
 	"DD_TRACE_AGENT_PORT":                                             {},
-	"DD_TRACE_V1_PAYLOAD_FORMAT_ENABLED":                              {},


this should also not get deleted (see below)! probably a rebase issue

I've now reset these files to main and rebased the branch and the tests are failing. The tests tell me to run go run ./scripts/configinverter/main.go generate and when I do it deletes this line.

See here: https://github.com/DataDog/dd-trace-go/runs/55310947575

I've put the changes back in order to pass tests.

It's not deleted, it's just moved below.

nice catch @jboolean I think the env var was actually manually added to both files without thinking this would break some stuff down the road.

sabrenner

mainly looked at mcpgo.go and double-checked the sdk changes to add the integration tag, looks good to me content-wise!

rarguelloF

LGTM!

jboolean

/merge

jboolean · 2025-11-13T17:06:17Z

/merge

dd-devflow-routing-codex · 2025-11-13T17:06:22Z

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2025-11-13 19:19:19 UTC ℹ️ MergeQueue: merge request added to the queue

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2025-11-13 19:39:29 UTC ℹ️ MergeQueue: This merge request was merged

github-actions bot added the apm:ecosystem contrib/* related feature requests or bugs label Nov 5, 2025

jboolean changed the title ~~Initial mcp-go tracer implementation~~ Initial mcp-go tracer implementation Nov 5, 2025

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feat(contrib/mcp-go): Trace MCP session initializations with MLObs spans #4101

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feat(contrib/mcp-go): Record MCP Session ID as tag #4106

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feat(contrib/mcp-go) Make simpler way to include mcp-go tracer #4115

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This was referenced Nov 13, 2025

fix: Fix duplicate and out-of-order key in supported_configurations #4121

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feat(contrib/mcp-go): Simplified way to add tracing #4122

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feat(contrib/mcp-go): Intent capture #4123

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Initial mcp-go tracer implementation

1c862b5

Fix double start of tracer in readme

91a4bc8

Comments in tests

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feat(contrib/mcp-go): Initial mcp-go tracer implementation #4100

feat(contrib/mcp-go): Initial mcp-go tracer implementation #4100

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What does this PR do?

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feat(contrib/mcp-go): Initial `mcp-go` tracer implementation #4100

feat(contrib/mcp-go): Initial `mcp-go` tracer implementation #4100

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