fix(passthrough): forward-proxy routes may claim reserved prefixes and mirror the whole path - #984
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…d mirror the whole path Running the real GitHub Copilot CLI (the agent product, not the IDE plugin) through a forward proxy surfaced two blockers. Its GitHub MCP server answers at /mcp/readonly on the very host it serves chat from, so a forward-proxy deployment must be able to route it: - The reserved-namespace rule rejected any /mcp, /v1, /a2a … prefix. That rule exists because the typed routes shadow a path-only route — but a route matching on `hosts` is dispatched by the host middleware wrapping the whole router, ahead of the typed routes, so its prefix is reachable. The rule now applies only to host-less routes, which keeps it from being used to shadow the gateway's own endpoints. - A `preserve_host` route stripped its path_prefix before forwarding, turning /mcp/readonly into /readonly and 404ing at the real backend. On a mirror route the prefix is a match condition, not a mount point: the upstream owns its path space. Stripping (and the /v1 dedup that rides on it) now applies only to target_url mounts. Verified against the live product: with both fixes the CLI's MCP session lifecycle relays cleanly through the gateway (POST 200/202, DELETE 204, GET 405 — byte-identical to direct), and the agent completes real tool-calling turns. Tests: schema coupling covers both directions of the reserved-prefix rule, match_route pins the mirrored path, and a new e2e claims /mcp on a host route end-to-end.
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📝 WalkthroughWalkthroughPassthrough routes with ChangesPassthrough routing
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The change enables host-matched passthrough routes to claim reserved prefixes and preserve the full upstream path, but the current end-to-end coverage does not actually validate the configured upstream and may pass on a 502, leaving a concrete routing regression insufficiently checked. Merge should wait for that test to be corrected; the public documentation also needs a minor clarification. Sequence Diagram(s)sequenceDiagram
participant Client
participant MatchedRoute
participant Upstream
Client->>MatchedRoute: Request host and /mcp/readonly
MatchedRoute->>MatchedRoute: Match host and /mcp without stripping the path
MatchedRoute->>Upstream: Forward /mcp/readonly
Upstream-->>Client: Return upstream response
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In `@crates/aisix-core/src/models/passthrough_route.rs`:
- Around line 27-34: Update the public API descriptions for the passthrough
route model in crates/aisix-core/src/models/passthrough_route.rs lines 27-34 and
schemas/resources/passthrough_route.schema.json line 542. Clarify that
target_url routes strip the matched path_prefix, while preserve_host routes use
it only for matching and forward the complete matched path; keep both
descriptions consistent.
In `@tests/e2e/src/cases/passthrough-route-e2e.test.ts`:
- Around line 317-366: Update the preserve_host E2E case around
createPassthroughRoute and call so it seeds the caller key after route creation,
uses the independently authenticated GET /v1/models propagation gate, and
requires a 200 response whose body contains routed: "mirrored". Validate the
mock upstream recorded the /mcp/readonly path and that x-aisix-api-key was
stripped; add Authorization only if needed to exercise forward_client.
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crates/aisix-core/src/models/passthrough_route.rscrates/aisix-proxy/src/passthrough_route.rsschemas/resources/passthrough_route.schema.jsontests/e2e/src/cases/passthrough-route-e2e.test.ts
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…ssert real behavior The path_prefix doc (and the regenerated schema description) now states the split explicitly: a target_url route MOUNTS at the prefix and strips it; a preserve_host route MIRRORS an upstream that owns its path space and forwards the complete path. The new e2e claimed to prove mirroring but could not: preserve_host dials https://<inbound host> and the mock listens on 127.0.0.1, so the request never reached it and the assertions were loose enough to pass anyway. It now pins what it can actually observe end-to-end — a host-matched route claiming the reserved /mcp namespace, reaching the upstream, and mounting normally — with a note pointing at the unit test and the live Copilot-CLI run that cover the mirroring half.
Running the real GitHub Copilot CLI — the standalone agent product, not the IDE plugin — through a forward proxy surfaced two blockers in the passthrough route model. The CLI reaches its GitHub MCP server at
/mcp/readonlyon the same host it serves chat inference from, so a forward-proxy deployment has to be able to route that.The reserved-namespace rule rejected the prefix.
path_prefixrefused/mcp,/v1,/a2a, … outright. The rule exists because the proxy's typed routes shadow a path-only route, making such a route unreachable by construction — but a route matching onhostsis dispatched by the host middleware that wraps the entire router, ahead of the typed routes, so its prefix is perfectly reachable. The rule now applies only to routes withouthosts, which keeps it doing its real job (nothing can shadow the gateway's own endpoints for ordinary traffic) while letting a forward proxy relay an upstream's own namespace.A
preserve_hostroute stripped its prefix before forwarding./mcp/readonlyleft the gateway as/readonlyand 404'd at the real backend. On a mirror route the prefix is a match condition, not a mount point — the upstream owns its path space. Stripping, and the/v1dedup that rides on the same flag, now apply only totarget_urlmounts, where an operator-written prefix really does join an operator-written base.Verified against the live product, not a mock: with both fixes the CLI's MCP session lifecycle relays cleanly through the gateway (POST
200/202, DELETE204, GET405— byte-identical to what it gets talking directly to GitHub), and the agent completes real tool-calling turns with usage and per-employee identity recorded on every span.Tests: the schema coupling table covers both directions of the reserved-prefix rule (host-less still rejected, host-matched accepted),
match_routepins that a mirrored path is relayed whole and never version-deduped, and a new e2e claims/mcpon a host route end-to-end.Refs api7/AISIX-Cloud#1312.
Summary by CodeRabbit
New Features
/mcp.Bug Fixes