feat(mcp): record input aliases used from a server-owned alias map by pauldambra · Pull Request #5117 · PostHog/posthog-js

pauldambra · 2026-09-25T19:45:30Z

Problem

MCP servers that accept alternative argument names (for example experimentId for id) want to know which spelling agents use. Without SDK support, each server adds its own $mcp_* property and format. Review on #5048 asked for this to be SDK work.

Changes

getToolInputProperties(args, schema, { inputAliases }) takes a server-owned map of canonical name to aliases, in the order the server tries them.
Alias names count as declared, so they stay visible in $mcp_input_keys.
New $mcp_input_aliases_used, for example ["experimentId:id"]: one entry for each canonical name the call did not send, using the first alias present. It is omitted when no alias was needed, and it is capped at 20. Argument values are never read.
New public type InputAliasMap. Documented in ARCHITECTURE.md.

Stacked on #5048. The PostHog MCP server will use this in place of its own $mcp_param_aliases_used (PostHog/posthog#101133).

Closes #5116

Release info Sub-libraries affected

Libraries affected

@posthog/mcp: minor release for the new option and property.

Checklist

Tests for new code
Accounted for the impact of any changes across different platforms
Accounted for backwards compatibility of any changes (no breaking changes!)
Took care not to unnecessarily increase the bundle size

Created with PostHog Desktop

🤖 Generated with Claude Code

Pass the schema's normalizeParamAliases map to getToolInputProperties as inputAliases, so the SDK records $mcp_input_aliases_used (alias:canonical) in place of the server's own $mcp_param_aliases_used (alias->canonical). describeAliasesUsed is removed; its rule tests now run through describeInputShape. Taxonomy, generated JSON, and the analytics vocabulary are updated. Depends on PostHog/posthog-js#5117 (stacked on #5048): CI fails until both are released and the @PostHog/mcp-analytics pin is bumped. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Generated-By: PostHog Desktop Task-Id: 8a3c5163-fb8f-423e-8008-17af786b8cae

chatgpt-codex-connector · 2026-09-25T19:52:08Z

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-25T19:54:21.337663Z	`5eab9ee`	Draft marked ready

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pauldambra · 2026-09-25T19:55:24Z

Note

🤖 Automated comment by QA Swarm — not written by a human

Multi-perspective review: router (cheap-first pass) + delegated lenses (qa-team, paul-reviewer, xp-reviewer, security-audit, engineering-systems-thinking as warranted)

Verdict: ✅ APPROVE _{(round 1 @ 5eab9ee)}

No actionable findings. inputAliases reads only key names (the getter test proves values are never read), uses hasOwnProperty for presence, keeps alias names under the existing 64-character and 20-entry caps, and changes nothing when the option is omitted.

Key findings

None.

Convergence

None (single reviewer).

Reviewer summaries

Reviewer	Assessment
🧭 router	No findings. Danger MEDIUM (public API addition to `@posthog/mcp`, but optional and additive), confidence HIGH. Nothing delegated. The `tool-input` tests pass (26).

Automated by QA Swarm — not a human review

pauldambra · 2026-09-25T20:04:11Z

@gesh this is the alias telemetry you asked for on #5048, built now instead of later. The server passes its own alias map as inputAliases. Alias names count as declared in $mcp_input_keys, and $mcp_input_aliases_used records alias:canonical only when the canonical name was not sent. It never reads values. It is stacked on #5048. This adds public API (inputAliases, InputAliasMap), so please confirm the shape here or on #5116. PostHog/posthog#101133 already uses it.

The PR was retargeted to a different base branch, so the approved diff is no longer what was reviewed. Stamphog re-reviews automatically.

getToolInputProperties takes inputAliases (canonical name to aliases). Alias names count as declared in $mcp_input_keys, and $mcp_input_aliases_used records alias:canonical for each canonical the call did not send, using the first alias present. Argument values are never read. Closes #5116 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Generated-By: PostHog Desktop Task-Id: 8a3c5163-fb8f-423e-8008-17af786b8cae

stamphog

Approved.

This adds a new public option/type to the MCP SDK, but it's optional, additive, well-tested (including a test proving argument values are never read), and the author has STRONG familiarity with this exact code (100% of touched lines, 52 merged PRs in these paths), which substitutes for a fresh independent review on risky public-API surface. No unresolved concerns in the reviews or discussion.

Author wrote 100% of the modified lines and has 52 merged PRs in these paths (familiarity STRONG).
👍 on the PR from chatgpt-codex-connector[bot].

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	45L, 4F substantive, 114L/7F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1c-medium (114L, 7F, two-areas, feat)
stamphog 2.2.0		`.stamphog/policy.yml` @ `02543de` · reviewed head `02543de`

Pass the schema's normalizeParamAliases map to getToolInputProperties as inputAliases, so the SDK records $mcp_input_aliases_used (alias:canonical) in place of the server's own $mcp_param_aliases_used (alias->canonical). describeAliasesUsed is removed; its rule tests now run through describeInputShape. Taxonomy, generated JSON, and the analytics vocabulary are updated. Depends on PostHog/posthog-js#5117 (stacked on #5048): CI fails until both are released and the @PostHog/mcp-analytics pin is bumped. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Generated-By: PostHog Desktop Task-Id: 8a3c5163-fb8f-423e-8008-17af786b8cae

pauldambra self-assigned this Sep 25, 2026

pauldambra added this pull request to stack #5049 September 25, 2026 19:45

pauldambra mentioned this pull request Sep 25, 2026

feat(mcp): record call shape, alias use, session order, and build on $mcp_tool_call PostHog/posthog#101133

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pauldambra marked this pull request as ready for review September 25, 2026 19:51

pauldambra requested review from a team as code owners September 25, 2026 19:51

pauldambra added the stamphog label Sep 25, 2026 — with PostHog

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pauldambra requested a review from gesh September 25, 2026 20:04

gesh approved these changes Sep 28, 2026

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gesh mentioned this pull request Sep 28, 2026

feat(mcp): resolve input aliases in automatic instrumentation #5130

Merged

6 tasks

Base automatically changed from posthog/mcp-safe-input-properties to main September 28, 2026 10:47

gesh force-pushed the posthog/mcp-input-aliases branch from 5eab9ee to 02543de Compare September 28, 2026 10:47

stamphog Bot approved these changes Sep 28, 2026

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gesh merged commit 51699b4 into main Sep 28, 2026
64 checks passed

gesh deleted the posthog/mcp-input-aliases branch September 28, 2026 11:04

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feat(mcp): record input aliases used from a server-owned alias map - #5117

feat(mcp): record input aliases used from a server-owned alias map#5117
gesh merged 1 commit into
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posthog/mcp-input-aliases

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pauldambra commented Sep 25, 2026

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pauldambra commented Sep 25, 2026

Verdict: ✅ APPROVE (round 1 @ 5eab9ee)

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pauldambra commented Sep 25, 2026

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chatgpt-codex-connector Bot commented Sep 25, 2026 •

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Verdict: ✅ APPROVE _{(round 1 @ 5eab9ee)}