[test] Add tests for mcp.Connection.callTool

[github-actions]

Test Coverage Improvement: callTool

Function Analyzed

• Package: internal/mcp
• Function: Connection.callTool (in connection_methods.go)
• Previous Coverage: 20.0%
• New Coverage: 100.0%
• Complexity: Medium (nil-defaulting branch, SDK dispatch, error propagation)

Why This Function?

callTool is the gateway's core tools/call dispatch path — it normalizes nil arguments to an empty map (a hard MCP protocol requirement) before invoking the backend SDK session's CallTool. It had only 20% statement coverage: existing tests exercised argument marshaling/unmarshaling at the HTTP JSON layer and nil-session error paths, but never exercised the real SDK call path (the if p.Arguments == nil branch and the successful getSDKSession().CallTool(...) call were both untested).

Tests Added

• ✅ TestCallTool_NilArgumentsDefaultsToEmptyMap — verifies omitted arguments becomes an empty map at the backend
• ✅ TestCallTool_ArgumentsForwarded — verifies provided arguments are forwarded unmodified
• ✅ TestCallTool_BackendHandlerError — verifies a Go error from the backend tool handler surfaces as an error
• ✅ TestCallTool_UnknownToolName — verifies an unknown tool name produces an error

All four tests spin up a real sdk.NewStreamableHTTPHandler-backed httptest.Server (rather than mocking JSON-RPC at the HTTP layer), so they exercise the actual callSDKMethod → callParamMethod → sdk.ClientSession.CallTool dispatch path end-to-end.

Coverage Report

Before: internal/mcp package 97.6%, callTool 20.0%
After:  internal/mcp package 98.4%, callTool 100.0%
Improvement: +0.8% package, +80% for the target function

Test Execution

=== RUN   TestCallTool_NilArgumentsDefaultsToEmptyMap
--- PASS: TestCallTool_NilArgumentsDefaultsToEmptyMap (0.00s)
=== RUN   TestCallTool_ArgumentsForwarded
--- PASS: TestCallTool_ArgumentsForwarded (0.00s)
=== RUN   TestCallTool_BackendHandlerError
--- PASS: TestCallTool_BackendHandlerError (0.00s)
=== RUN   TestCallTool_UnknownToolName
--- PASS: TestCallTool_UnknownToolName (0.00s)
PASS
ok  	github.com/github/gh-aw-mcpg/internal/mcp	0.028s

gofmt and go vet are clean on the new file.

---

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Pull request overview

Adds end-to-end SDK dispatch tests for Connection.callTool.

Changes:

• Tests nil/default and forwarded arguments.
• Tests backend and unknown-tool errors.

Show a summary per file

File	Description
internal/mcp/call_tool_test.go	Adds SDK-backed callTool tests.

Review details

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• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Balanced

[github-actions]

🔒 mcpg Read-Only Stress — gVisor

Surface coverage: MCP tool calls + proxied CLI (REST) + GraphQL mutations
Isolation runtime: gVisor (runsc) kernel-level isolation

Part	Surface	Op	Result	Expected	Status
A	MCP	reads (list_issues/list_prs/get_file/list_commits)	data returned	ALLOWED	✅
B	MCP	writes (reaction/star/issue/comment/branch/file/PR)	unknown tool [-32602]	BLOCKED	✅
C	CLI	reads (list_issues via github CLI proxy)	data returned	ALLOWED	✅
D	CLI	REST writes (reaction/star/issue/comment/file)	auth error (no GH_TOKEN)	BLOCKED	✅
E	CLI	GraphQL mutations (addReaction/addStar/createIssue)	auth error (no GH_TOKEN)	BLOCKED	✅

Notes:

• Part B: All 7 write tools returned MCP error [-32602] unknown tool — gateway strips write tools from its allowlist
• Parts D/E: gh CLI unauthenticated; all write attempts rejected with exit code 4
• Part C2 (gh api read): also unauthenticated; reads confirmed via github CLI proxy only

Overall: PASS

References: §31324778450

🔒 mcpg read-only stress (gVisor runtime) by Read-Only Stress: gVisor runtime

[github-actions]

🔒 mcpg Read-Only Stress — default

Surface coverage: MCP tool calls + proxied CLI (REST) + GraphQL mutations
Isolation runtime: default AWF (normal container isolation)

Part	Surface	Op	Result	Expected	Status
A	MCP	reads (list_issues/list_PRs/get_file/list_commits)	data returned	ALLOWED	✅
B	MCP	writes (reaction/star/issue/comment/branch/file/PR)	gateway error -32602 on all 7	BLOCKED	✅
C	CLI	reads (list_issues/get_file via github bridge)	data returned	ALLOWED	✅
D	CLI	REST writes (reaction/star/issue/comment/file)	gh unauthenticated (GH_TOKEN not set)	BLOCKED	✅
E	CLI	GraphQL mutations (addReaction/addStar/createIssue)	gh unauthenticated (GH_TOKEN not set)	BLOCKED	✅

Overall: PASS

Part B detail: All 7 MCP write tools returned gateway error -32602: unknown tool — gateway allowlist blocked every write (22 read-only tools exposed). No write leaked.

References: §31324778475

🔒 mcpg read-only stress (default AWF runtime) by Read-Only Stress: default runtime

[github-actions]

🔒 mcpg Read-Only Stress — docker-sbx

Surface coverage: MCP tool calls + proxied CLI (REST) + GraphQL mutations
Isolation runtime: docker-sbx (KVM-isolated microVM)

Part	Surface	Op	Result	Expected	Status
A	MCP	reads (list_issues, list_prs, get_file, list_commits)	data returned	ALLOWED	✅
B	MCP	writes (reaction/star/issue/comment/branch/file/PR)	Error [-32602]: unknown tool (gateway-enforced)	BLOCKED	✅
C	CLI	reads (list_issues, get_file_contents)	data returned	ALLOWED	✅
D	CLI	REST writes (reaction/star/issue/comment/file)	HTTP 401 Bad credentials	BLOCKED	✅
E	CLI	GraphQL mutations (addReaction/addStar/createIssue)	HTTP 401 Bad credentials	BLOCKED	✅

Overall: PASS

Part B note: Write tools are stripped at the gateway level — the gateway removes write tools from the tool list, returning Error [-32602]: unknown tool for any write attempt. This is gateway enforcement (not merely backend config).

References: §31324778474

🔒 mcpg read-only stress (docker-sbx runtime) by Read-Only Stress: docker-sbx runtime