feat(filter): add identity header guard filter

[yossiovadia]

Summary

• Add identity_header_guard filter that captures headers matching a configurable prefix into filter_metadata and strips them before upstream forwarding
• Prevents identity headers (e.g. x-tenant-username, x-tenant-group) from leaking to LLM providers
• Captured values are available to downstream filters like external_metering (feat(filter): add external metering filter for usage reporting and balance checks #577)

Fixes #698

Motivation

The external_metering filter reads tenant identity from request headers for per-user usage attribution. These headers are set by an upstream auth layer and must not reach the upstream provider. reserved_headers in core only handles hardcoded x-praxis-* prefixes with no metadata capture and no configurable prefixes (core TODO #186).

Design

• Configurable header prefix (default: x-tenant-)
• Captured headers written to filter_metadata under a configurable namespace (prevents collision with verified auth metadata)
• Matched headers marked for removal via request_headers_to_remove
• ~120 lines of filter code

What's included

• filters/src/identity_guard/ — filter, config, 11 unit tests + 1 doctest
• tests/integration/tests/suite/examples/identity_header_guard.rs — 3 integration tests (config parse + header capture + strip)
• examples/configs/identity-header-guard.yaml — example config
• Generated filter docs and README updates

Test plan

• 11 unit tests covering: prefix matching, case insensitivity, namespace isolation, no-match passthrough, empty/missing headers, multiple captures, strip verification
• 1 doctest
• 3 integration tests (config parse, header capture to metadata, upstream strip)
• cargo xtask lint-example-tests — passes (example config has test coverage)
• cargo xtask lint-filter-docs — passes (generated docs up to date)
• cargo clippy -p praxis-ai-filters -- -D warnings — zero warnings
• Validated end-to-end on OpenShift deployment with external_metering consuming captured identity

[leseb]

would core be more suitable for this instead of this repo? it's not so much "ai" related

[praxis-bot]

Clean implementation — the filter logic is correct and the security-critical behavior (stripping before forwarding, namespaced metadata to avoid collision with verified auth) is well thought out. Three medium findings, all related to test coverage gaps.

[jordigilh]

Flagging a likely conflict: this filter's x-tenant- prefix capture-and-strip overlaps with #581's external_metering filter, which expects the same x-tenant-* header convention (username/group/subscription/model) and does its own stripping of those headers. Neither PR references the other.

If both land as-is, the pipeline would end up with two filters independently capturing/stripping the same headers into different metadata shapes. Might be worth the two of you syncing on which owns the canonical filter_metadata namespace before either merges — cc @noyitz.

[praxis-bot]

One new medium finding on duplicate-header handling. The previous review's three findings (misleading default-namespace test, missing registry assertion, non-UTF-8 edge case) still apply and are not repeated here.

[leseb]

@yossiovadia please address the bot's review or dismiss with a reason

[yossiovadia]

Good catch @jordigilh. This filter and #581 are designed to work together, not independently:

• identity_header_guard runs before external_metering in the pipeline. It captures x-tenant-* headers into namespaced metadata (identity.x-tenant-username) and strips them.
• external_metering (feat(metering): add external metering filter with balance checks and usage reporting #581) then reads identity from filter_metadata (where this filter put it) rather than from raw request headers. This is the trusted path — metadata written by an earlier filter is verified, headers are not.

The overlap in stripping is intentional redundancy: if identity_header_guard is present, it strips first. If it's absent (e.g., a deployment that doesn't need identity isolation), external_metering handles it as fallback.

The canonical metadata namespace is identity. (configurable via metadata_namespace). #581's metering filter reads from that namespace first, falling back to raw headers only when the guard isn't in the pipeline.

cc @noyitz — this is the same pattern as IPP's maas-headers-guard → external-metering dependency.

[aslakknutsen]

Any reason why this feature couldn't be expressed by using the core filter Header? https://github.com/praxis-proxy/praxis/blob/main/filter/src/builtins/http/transformation/header/mod.rs#L82

Potentially adding wildcard and "move to metadata" as a feature?

@alexsnaps

[yossiovadia]

Good question. Looked at the core HeaderFilter — it supports exact-name request_remove, request_set, etc. What this filter adds:

1. Prefix matching — matches by configurable prefix (x-tenant-*), not exact header names. The set of headers isn't known at config time — an upstream auth layer (Authorino, Keycloak) can inject any x-tenant-* header.
2. Capture to metadata — writes values to filter_metadata before stripping. The core filter strips headers but doesn't preserve them anywhere. Downstream filters (metering, audit) need the captured values.
3. Namespaced metadata keys — writes to identity.x-tenant-username, not x-tenant-username, to prevent collision with auth-verified metadata from jwt_auth or api_key_auth.

Could these be added to the core filter? Yes — as a request_capture_and_remove option with a prefix field and metadata namespace. That's a valid design. The trade-off:

• Extending core: more capable core filter, but adds identity/security semantics to a general-purpose transformation filter. The "capture to namespaced metadata then strip" behavior is a security invariant, not a transformation — getting the ordering wrong leaks identity to providers.
• Separate filter: smaller (~80 lines), self-contained, security intent is explicit in the name and placement. Easier to audit.

I'm fine either way — if the team prefers this as a core headers extension, I can refactor. The filter_metadata write + namespace isolation are the non-negotiable parts regardless of where the code lives.

cc @alexsnaps

[jordigilh]

CI's green but there's a merge conflict with main — mind rebasing? This blocks #130/#104.

Also still open: the standalone-vs-core-filter question from @aslakknutsen (Aug 13) — needs a call from @alexsnaps.

[yossiovadia]

Rebased onto latest main — conflicts were just the filter-registration list (register.rs import + the registry test's expected-filters array) and the integration examples/mod.rs module list, all from filters that landed on main since this branched. Resolved to the superset; cargo test, clippy, and fmt are clean locally and the net diff is unchanged in scope (11 files, identity_header_guard only). Should unblock #130/#104.

Intent is unchanged from the original PR: this stays the self-contained temporary bridge @leseb green-lit in #708, writing captured identity to filter_metadata (not upstream headers), with nothing else depending on it.

On the standalone-vs-core-filter question from @aslakknutsen (Aug 13): still needs a call from @alexsnaps. I'm happy to refactor into a core headers extension instead if that's the direction — the metadata-capture + strip behavior is the only non-negotiable; where the code lives is the maintainers' call.

[praxis-bot]

Re-review (3/3)

Previous findings #2 (registry assertion), #3 (non-UTF-8 test), and #4 (first-wins semantics) are resolved. One new finding from the fix commits.

[praxis-bot]

One new medium finding on untested validation logic. All previous findings are resolved.

[jordigilh]

Approving — the last-wins vs first-wins security gap is fixed (duplicate_headers_first_value_wins, a6bca42), matching the bot's original recommendation.

[praxis-bot]

Re-review (5/5)

One new medium finding. The previous finding (#6, empty-namespace validation test) is still open and not repeated here.

[leseb]

@yossiovadia please rebase

[yossiovadia]

Rebased onto current main (post-#581). Conflict resolution was limited to the three registration sites (register.rs import list, examples/README.md table, integration suite mod.rs) — all both-sides-added-a-line merges, verified against the xtask sync checks. All gates green locally: filter unit + doctests + integration example, clippy (both feature sets), nightly fmt, machete, xtask suite.

One note for reviewers: this force-push also removes four previously stacked, now-superseded commits (MCP metadata block / credential redaction, #436/#438/#578, plus a chores commit) — upstream reimplemented that redaction/SSRF work in faa043f. The PR now contains only the identity_header_guard work.

@shaneutt fresh eyes when you get a chance — the rebase supersedes the earlier review.

[noyitz]

@leseb can you confirm and approve this one?