diff --git a/changelog.d/fixes/10225-combo-context-overflow-before-compression.md b/changelog.d/fixes/10225-combo-context-overflow-before-compression.md
new file mode 100644
index 000000000000..0a678180afd1
--- /dev/null
+++ b/changelog.d/fixes/10225-combo-context-overflow-before-compression.md
@@ -0,0 +1 @@
+- **fix(combo):** defer the known-context-overflow hard rejection for compressible requests so compression runs before the final context gate, instead of a raw-body estimate 400'ing generic Responses clients targeting a large model before OmniRoute can shrink it ([#10225](https://github.com/diegosouzapw/OmniRoute/issues/10225))
\ No newline at end of file
diff --git a/open-sse/services/combo.ts b/open-sse/services/combo.ts
index df86a79375d7..46b1f743baa3 100644
--- a/open-sse/services/combo.ts
+++ b/open-sse/services/combo.ts
@@ -591,6 +591,11 @@ export async function handleComboChat({
   nesting = null,
   hiddenModelsByProvider = getHiddenModelsByProvider(),
   clientManagedResponsesContext = false,
+  deferContextOverflowWhenCompressible = false,
+  compressionExclusions,
+  sourceFormat = null,
+  endpointPath = null,
+  requestHeaders = null,
 }: HandleComboChatOptions): Promise<Response> {
   const comboCtx = createComboContext({ body, combo, settings, relayOptions, log });
   const {
@@ -651,6 +656,11 @@ export async function handleComboChat({
     signal,
     apiKeyAllowedConnections,
     hiddenModelsByProvider,
+    deferContextOverflowWhenCompressible,
+    compressionExclusions,
+    sourceFormat,
+    endpointPath,
+    requestHeaders,
     runCombo: handleComboChat,
   });
   if (fusionDispatch) return fusionDispatch;
@@ -700,6 +710,11 @@ export async function handleComboChat({
     signal,
     apiKeyAllowedConnections,
     hiddenModelsByProvider,
+    deferContextOverflowWhenCompressible,
+    compressionExclusions,
+    sourceFormat,
+    endpointPath,
+    requestHeaders,
     runCombo: handleComboChat,
   });
   if (runtimeUnitDispatch) return runtimeUnitDispatch;
@@ -723,6 +738,11 @@ export async function handleComboChat({
       signal,
       hiddenModelsByProvider,
       clientManagedResponsesContext,
+      deferContextOverflowWhenCompressible,
+      compressionExclusions,
+      sourceFormat,
+      endpointPath,
+      requestHeaders,
       relayOptions,
     });
   }
@@ -750,6 +770,11 @@ export async function handleComboChat({
     buildAutoCandidates,
     hiddenModelsByProvider,
     clientManagedResponsesContext,
+    deferContextOverflowWhenCompressible,
+    compressionExclusions,
+    sourceFormat,
+    endpointPath,
+    requestHeaders,
   });
   if ("earlyResponse" in targetResolution) return targetResolution.earlyResponse;
   const { stickyWeightedLimit, getWeightedStepKeyForTarget, preScreenMap } = targetResolution;
@@ -2441,6 +2466,11 @@ async function handleRoundRobinCombo({
   nesting = null,
   hiddenModelsByProvider = getHiddenModelsByProvider(),
   clientManagedResponsesContext,
+  deferContextOverflowWhenCompressible = false,
+  compressionExclusions,
+  sourceFormat = null,
+  endpointPath = null,
+  requestHeaders = null,
   relayOptions,
 }: HandleRoundRobinOptions): Promise<Response> {
   const config = settings
@@ -2498,6 +2528,11 @@ async function handleRoundRobinCombo({
   const evalRankedTargets = orderTargetsByEvalScores(tagFilteredTargets, config.evalRouting, log);
   const knownContextOverflow = getKnownContextOverflow(evalRankedTargets, body, {
     clientManagedResponsesContext,
+    deferContextOverflowWhenCompressible,
+    compressionExclusions,
+    sourceFormat,
+    endpointPath,
+    requestHeaders,
   });
   if (knownContextOverflow) {
     return errorResponseWithComboDiagnostics(
diff --git a/open-sse/services/combo/dispatchPrelude.ts b/open-sse/services/combo/dispatchPrelude.ts
index 7caf82fe7653..a24eb1a46833 100644
--- a/open-sse/services/combo/dispatchPrelude.ts
+++ b/open-sse/services/combo/dispatchPrelude.ts
@@ -76,6 +76,14 @@ type PreludeBaseOptionArgs = {
   apiKeyAllowedConnections?: string[] | null;
   hiddenModelsByProvider?: HiddenModelsByProvider;
   clientManagedResponsesContext?: boolean;
+  /** #10225 — defer the hard context-overflow preflight when compression is enabled. */
+  deferContextOverflowWhenCompressible?: boolean;
+  /** Server-side compression exclusions (#8034). */
+  compressionExclusions?: import("../compression/exclusions.ts").CompressionExclusions;
+  /** #10503 — request-shape facts for the target-aware deferral check (see knownContextOverflow.ts). */
+  sourceFormat?: string | null;
+  endpointPath?: string | null;
+  requestHeaders?: Headers | Record<string, unknown> | null;
 };
 
 /** Rebuild handleComboChat's option bag verbatim for a recursive dispatch. */
@@ -93,6 +101,11 @@ function buildBaseOptions(a: PreludeBaseOptionArgs): HandleComboChatOptions {
     apiKeyAllowedConnections: a.apiKeyAllowedConnections,
     hiddenModelsByProvider: a.hiddenModelsByProvider,
     clientManagedResponsesContext: a.clientManagedResponsesContext,
+    deferContextOverflowWhenCompressible: a.deferContextOverflowWhenCompressible,
+    compressionExclusions: a.compressionExclusions,
+    sourceFormat: a.sourceFormat,
+    endpointPath: a.endpointPath,
+    requestHeaders: a.requestHeaders,
   };
 }
 
@@ -366,6 +379,11 @@ export async function tryFusionDispatch(args: {
   signal?: AbortSignal | null;
   apiKeyAllowedConnections?: string[] | null;
   hiddenModelsByProvider?: HiddenModelsByProvider;
+  deferContextOverflowWhenCompressible?: boolean;
+  compressionExclusions?: import("../compression/exclusions.ts").CompressionExclusions;
+  sourceFormat?: string | null;
+  endpointPath?: string | null;
+  requestHeaders?: Headers | Record<string, unknown> | null;
   runCombo: RunCombo;
 }): Promise<Response | null> {
   const { cfg, combo, config, strategy, log } = args;
@@ -589,6 +607,11 @@ export async function tryRuntimeUnitDispatch(args: {
   signal?: AbortSignal | null;
   apiKeyAllowedConnections?: string[] | null;
   hiddenModelsByProvider?: HiddenModelsByProvider;
+  deferContextOverflowWhenCompressible?: boolean;
+  compressionExclusions?: import("../compression/exclusions.ts").CompressionExclusions;
+  sourceFormat?: string | null;
+  endpointPath?: string | null;
+  requestHeaders?: Headers | Record<string, unknown> | null;
   runCombo: RunCombo;
 }): Promise<Response | null> {
   const { body, combo, config, strategy, allCombos, log, settings } = args;
diff --git a/open-sse/services/combo/knownContextOverflow.ts b/open-sse/services/combo/knownContextOverflow.ts
index db9cb2d602e2..532e1a4be616 100644
--- a/open-sse/services/combo/knownContextOverflow.ts
+++ b/open-sse/services/combo/knownContextOverflow.ts
@@ -17,6 +17,8 @@
  */
 
 import { getResolvedModelCapabilities } from "../modelCapabilities.ts";
+import { isCompressionExcluded, type CompressionExclusions } from "../compression/exclusions.ts";
+import { shouldUseNativeCodexPassthrough } from "../../handlers/chatCore/passthroughHelpers.ts";
 import { deriveRequestCompatibilityRequirements } from "./comboStructure.ts";
 import type { ResolvedComboTarget } from "./types.ts";
 
@@ -28,6 +30,29 @@ export type KnownContextOverflow = {
   targetCount: number;
 };
 
+export type KnownContextOverflowOptions = {
+  clientManagedResponsesContext?: boolean;
+  /**
+   * When prompt compression is enabled for this request (global compression switch
+   * AND not API-key opted-out), defer the hard preflight so chatCore's compression
+   * pipeline runs before the final context gate — instead of a raw-body estimate
+   * rejecting a compressible request up front. (#10225)
+   */
+  deferContextOverflowWhenCompressible?: boolean;
+  /** Server-side compression exclusions (#8034) — targets matching one cannot run compression. */
+  compressionExclusions?: CompressionExclusions;
+  /**
+   * #10503: the exact request-shape facts chatCore.ts uses to decide
+   * `shouldUseNativeCodexPassthrough` (open-sse/handlers/chatCore/passthroughHelpers.ts) —
+   * threaded down so the deferral decision below can be target-aware instead of
+   * relying on the looser `clientManagedResponsesContext` proxy. Reused verbatim
+   * (not re-derived) so the combo-layer decision can never drift from chatCore's own.
+   */
+  sourceFormat?: string | null;
+  endpointPath?: string | null;
+  requestHeaders?: Headers | Record<string, unknown> | null;
+};
+
 // #7177: an empty array/object (e.g. a default `messages: []` some combo entrypoints inject
 // when the caller sent none) has no real content — counting it would charge a few phantom
 // "structural" tokens (JSON.stringify braces/brackets) toward the estimate, which is enough
@@ -69,7 +94,7 @@ export function getKnownContextLimit(
 export function getKnownContextOverflow(
   targets: ResolvedComboTarget[],
   body: Record<string, unknown>,
-  options: { clientManagedResponsesContext?: boolean } = {}
+  options: KnownContextOverflowOptions = {}
 ): KnownContextOverflow | null {
   if (targets.length === 0) return null;
   // Native Codex Responses clients compact their own item history. Let the concrete
@@ -85,6 +110,55 @@ export function getKnownContextOverflow(
   ) {
     return null;
   }
+  // #10225 / #10499-sweep #10503: a conservative raw-body context estimate must not
+  // be treated as proof that a compression-enabled request cannot fit. When
+  // compression is available for this request AND at least one target can actually
+  // run it, defer the hard rejection so handleChatCore runs proactive compression
+  // (chatCore.ts) and its post-compression enforceOutputTokenBudget becomes the
+  // final context gate — returning a local `context_length_exceeded` only if the
+  // compressed body still cannot fit (no upstream dispatch).
+  //
+  // Target-awareness is load-bearing here: a target is only a valid reason to defer
+  // when handleChatCore will ACTUALLY attempt compression for it. Two classes are
+  // excluded from "can compress" even though `isCompressionExcluded` (operator
+  // exclusions) says nothing about them:
+  //   - Operator-excluded targets (#8034, existing `isCompressionExcluded` check).
+  //   - Native Codex Responses passthrough targets: chatCore.ts unconditionally sets
+  //     `compressionExcluded = nativeCodexPassthrough || ...` for these, computed via
+  //     `shouldUseNativeCodexPassthrough()` (chatCore/passthroughHelpers.ts) — called
+  //     here with the SAME request-shape facts (sourceFormat/endpointPath/headers)
+  //     chatCore itself uses, reused verbatim rather than re-derived from the looser
+  //     `clientManagedResponsesContext` flag (which always requires a VERIFIED native
+  //     client; chatCore's own gate does NOT for provider==="codex" — see
+  //     shouldUseNativeCodexPassthrough's `provider === "codex" || isVerifiedNativeCodexRequest`
+  //     short-circuit). Deferring on such a target's account would let an oversized
+  //     body sail straight through to `fetch()` uncompressed instead of being caught
+  //     by either preflight — silently defeating the whole point of this feature.
+  // If NO target can compress, the fast raw-body preflight is kept (unchanged).
+  if (
+    options.deferContextOverflowWhenCompressible === true &&
+    targets.some((target) => {
+      const isNativeCodexPassthroughTarget = shouldUseNativeCodexPassthrough({
+        provider: target.provider,
+        sourceFormat: options.sourceFormat,
+        endpointPath: options.endpointPath,
+        body,
+        headers: options.requestHeaders,
+      });
+      if (isNativeCodexPassthroughTarget) return false;
+      return !isCompressionExcluded(
+        {
+          provider: target.provider,
+          model: target.modelStr.includes("/")
+            ? target.modelStr.split("/").slice(1).join("/")
+            : target.modelStr,
+        },
+        options.compressionExclusions
+      );
+    })
+  ) {
+    return null;
+  }
   const requirements = deriveRequestCompatibilityRequirements(body);
   if (requirements.requiredContextTokens <= 0) return null;
 
diff --git a/open-sse/services/combo/targetResolution.ts b/open-sse/services/combo/targetResolution.ts
index d5af5a6e01f2..45aa57013fc2 100644
--- a/open-sse/services/combo/targetResolution.ts
+++ b/open-sse/services/combo/targetResolution.ts
@@ -115,6 +115,14 @@ export interface ResolveComboTargetPipelineDeps {
   hiddenModelsByProvider?: HiddenModelsByProvider;
   /** Native Responses clients (for example Codex CLI/Desktop) manage compaction themselves. */
   clientManagedResponsesContext?: boolean;
+  /** #10225 — defer the hard context-overflow preflight when compression is enabled for this request. */
+  deferContextOverflowWhenCompressible?: boolean;
+  /** Server-side compression exclusions (#8034) — which targets can run compression. */
+  compressionExclusions?: import("../compression/exclusions.ts").CompressionExclusions;
+  /** #10503 — request-shape facts for the target-aware deferral check (see knownContextOverflow.ts). */
+  sourceFormat?: string | null;
+  endpointPath?: string | null;
+  requestHeaders?: Headers | Record<string, unknown> | null;
 }
 
 export interface ResolvedComboTargetPipeline {
@@ -730,6 +738,11 @@ export async function resolveComboTargetPipeline(
 
   const overflow = getKnownContextOverflow(orderedTargets, body, {
     clientManagedResponsesContext: deps.clientManagedResponsesContext,
+    deferContextOverflowWhenCompressible: deps.deferContextOverflowWhenCompressible,
+    compressionExclusions: deps.compressionExclusions,
+    sourceFormat: deps.sourceFormat,
+    endpointPath: deps.endpointPath,
+    requestHeaders: deps.requestHeaders,
   });
   if (overflow) {
     return { earlyResponse: buildContextOverflowResponse(overflow, orderedTargets, log) };
diff --git a/open-sse/services/combo/types.ts b/open-sse/services/combo/types.ts
index 9f9f31c4b45b..3a9ad2c010b8 100644
--- a/open-sse/services/combo/types.ts
+++ b/open-sse/services/combo/types.ts
@@ -6,6 +6,7 @@
  * — logic unchanged, re-exported from combo.ts for backward compatibility.
  */
 
+import type { CompressionExclusions } from "../compression/exclusions.ts";
 import type { ProviderCandidate } from "../autoCombo/scoring.ts";
 
 export const RESET_WINDOW_NAMES = ["weekly", "session", "monthly"] as const;
@@ -112,6 +113,25 @@ export type HandleComboChatOptions = {
   hiddenModelsByProvider?: HiddenModelsByProvider;
   /** Native Responses clients (for example Codex CLI/Desktop) manage compaction themselves. */
   clientManagedResponsesContext?: boolean;
+  /**
+   * #10225: request-scoped flag — prompt compression is enabled for this request
+   * (global compression switch ON and not opted-out by the API key). When set, the
+   * combo preflight defers its hard context-overflow rejection so chatCore's
+   * compression runs before the final context gate.
+   */
+  deferContextOverflowWhenCompressible?: boolean;
+  /** Server-side compression exclusions (#8034) — used to check which targets can run compression. */
+  compressionExclusions?: CompressionExclusions;
+  /**
+   * #10503: request-shape facts (mirroring chatCore.ts's own resolution) threaded
+   * down to getKnownContextOverflow so the deferral decision can be target-aware —
+   * a native-Codex-Responses-passthrough target must never count as "compressible"
+   * (chatCore disables compression for it unconditionally). See
+   * knownContextOverflow.ts::KnownContextOverflowOptions for the full rationale.
+   */
+  sourceFormat?: string | null;
+  endpointPath?: string | null;
+  requestHeaders?: Headers | Record<string, unknown> | null;
 };
 
 export type HandleRoundRobinOptions = Omit<HandleComboChatOptions, "apiKeyAllowedConnections">;
diff --git a/src/sse/handlers/chat.ts b/src/sse/handlers/chat.ts
index 9002225a1b37..e8239d3835e4 100644
--- a/src/sse/handlers/chat.ts
+++ b/src/sse/handlers/chat.ts
@@ -33,6 +33,8 @@ import type { SingleModelTarget } from "@omniroute/open-sse/services/combo/types
 import { mergeAbortSignals } from "@omniroute/open-sse/executors/base.ts";
 import { resolveRequestAutoControls } from "@omniroute/open-sse/services/autoCombo/requestControls.ts";
 import { isVerifiedNativeCodexRequest } from "@omniroute/open-sse/config/codexIdentity.ts";
+import { resolveCompressionSettings } from "@omniroute/open-sse/handlers/chatCore/compressionSettings.ts";
+import type { CompressionExclusions } from "@omniroute/open-sse/services/compression/exclusions.ts";
 import { resolveComboConfig } from "@omniroute/open-sse/services/comboConfig.ts";
 import { injectHandoffIntoBody } from "@omniroute/open-sse/services/contextHandoff.ts";
 import {
@@ -209,6 +211,31 @@ let combosCacheTs = 0;
 let combosCacheVersionSnapshot = -1;
 const COMBOS_CACHE_TTL_MS = 10_000;
 
+/**
+ * #10225 — resolve whether this request's combo preflight should DEFER its hard
+ * context-overflow rejection so chatCore's compression runs first.
+ *
+ * Mirrors handleChatCore's own enablement determination (chatCore.ts): defer only
+ * when the global compression switch is ON and the API key has not opted out
+ * (`apiKeyInfo.compressionEnabled !== false`). Per-target applicability (server-side
+ * exclusions) is checked inside getKnownContextOverflow via the returned exclusions.
+ * Fail closed (defer=false) on any lookup error — the existing hard preflight stays.
+ */
+async function resolveComboContextOverflowDeferral(
+  logger: { warn?: (...args: unknown[]) => void } | null | undefined,
+  apiKeyInfo: { compressionEnabled?: boolean } | null | undefined
+): Promise<{ defer: boolean; exclusions: CompressionExclusions | undefined }> {
+  try {
+    const compression = await resolveCompressionSettings(logger);
+    return {
+      defer: compression.enabled && apiKeyInfo?.compressionEnabled !== false,
+      exclusions: compression.settings?.exclusions,
+    };
+  } catch {
+    return { defer: false, exclusions: undefined };
+  }
+}
+
 async function getCombosCachedForChat(): Promise<unknown[]> {
   const now = Date.now();
   // Explicit non-null check: we intentionally cache and return the Promise
@@ -824,9 +851,20 @@ async function handleChatImplementation(
 
     // Context-relay keeps generation in combo.ts, but handoff injection lives here
     // because only this layer knows which connectionId was actually selected.
+    const { defer: deferContextOverflowWhenCompressible, exclusions: compressionExclusions } =
+      await resolveComboContextOverflowDeferral(log, apiKeyInfo);
     const response = await (handleComboChat as any)({
       body,
       combo,
+      deferContextOverflowWhenCompressible,
+      compressionExclusions,
+      // #10503: same request-shape facts chatCore.ts resolves for itself
+      // (resolveChatCoreRequestFormat), so getKnownContextOverflow's target-aware
+      // deferral check can never drift from chatCore's own native-codex-passthrough
+      // decision. See knownContextOverflow.ts::KnownContextOverflowOptions.
+      sourceFormat,
+      endpointPath: new URL(request.url).pathname,
+      requestHeaders: request.headers,
       clientManagedResponsesContext:
         sourceFormat === "openai-responses" &&
         new URL(request.url).pathname.split("/").includes("responses") &&
@@ -1103,11 +1141,22 @@ async function handleSingleModelChat(
     );
     log.info("ROUTING", `Auto-combo redirect from handleSingleModelChat for "${modelStr}"`);
     log.info("ROUTING", `Auto-combo redirect to combo flow for "${modelStr}"`);
+    const { defer: sNetDefer, exclusions: sNetExclusions } =
+      await resolveComboContextOverflowDeferral(log, apiKeyInfo);
+    // #10503: same request-shape facts chatCore.ts resolves for itself — threaded
+    // down so getKnownContextOverflow's target-aware deferral check can never drift
+    // from chatCore's own native-codex-passthrough decision.
+    const sNetSourceFormat = detectFormatFromEndpoint(body, clientRawRequest?.endpoint || "");
     return handleComboChat({
       body,
       combo: redirectCombo,
+      deferContextOverflowWhenCompressible: sNetDefer,
+      compressionExclusions: sNetExclusions,
+      sourceFormat: sNetSourceFormat,
+      endpointPath: clientRawRequest?.endpoint || "",
+      requestHeaders: clientRawRequest?.headers,
       clientManagedResponsesContext:
-        detectFormatFromEndpoint(body, clientRawRequest?.endpoint || "") === "openai-responses" &&
+        sNetSourceFormat === "openai-responses" &&
         String(clientRawRequest?.endpoint || "")
           .split("/")
           .includes("responses") &&
diff --git a/tests/unit/combo-context-overflow-compression-probe.test.ts b/tests/unit/combo-context-overflow-compression-probe.test.ts
new file mode 100644
index 000000000000..f482cba3524f
--- /dev/null
+++ b/tests/unit/combo-context-overflow-compression-probe.test.ts
@@ -0,0 +1,409 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+/**
+ * #10225 — combo known-context-overflow must NOT hard-reject a compressible
+ * request before OmniRoute's compression pipeline can run.
+ *
+ * Root cause: getKnownContextOverflow() estimates the RAW body (ceil(serializedChars/4)
+ * over the whole Responses input[]) during combo target resolution, before any
+ * compression. When every known target limit is below that raw estimate, both call
+ * sites (round-robin + target-resolution) convert it into an immediate local 400
+ * `context_length_exceeded` with attempted:0 — so chatCore's proactive compression
+ * (which can shrink 294133→111529, 62% in the reporter's case) never runs. The only
+ * existing bypass (clientManagedResponsesContext) is gated to VERIFIED native Codex
+ * clients, so a generic Responses client (e.g. OpenCode) pointed at a codex model
+ * still hits the hard gate.
+ *
+ * Fix: thread a request-scoped `deferContextOverflowWhenCompressible` flag (set when
+ * the global compression switch is ON and not API-key opted-out). When set AND at
+ * least one target can run compression, getKnownContextOverflow returns null so the
+ * request reaches chatCore, whose post-compression enforceOutputTokenBudget becomes
+ * the final context gate — a local 400 only if the compressed body still cannot fit.
+ */
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-combo-overflow-compress-"));
+const ORIGINAL_DATA_DIR = process.env.DATA_DIR;
+process.env.DATA_DIR = TEST_DATA_DIR;
+
+const core = await import("../../src/lib/db/core.ts");
+const { saveModelsDevCapabilities, clearModelsDevCapabilities } =
+  await import("../../src/lib/modelsDevSync.ts");
+const { getKnownContextOverflow, handleComboChat } = await import(
+  "../../open-sse/services/combo.ts"
+);
+const { updateCompressionSettings } = await import("../../src/lib/db/compression.ts");
+const { handleChatCore } = await import("../../open-sse/handlers/chatCore.ts");
+
+test.after(() => {
+  core.resetDbInstance();
+  if (ORIGINAL_DATA_DIR === undefined) {
+    delete process.env.DATA_DIR;
+  } else {
+    process.env.DATA_DIR = ORIGINAL_DATA_DIR;
+  }
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+test.beforeEach(() => {
+  clearModelsDevCapabilities();
+});
+
+function capabilityEntry(limitContext: number | null) {
+  return {
+    tool_call: true,
+    reasoning: false,
+    attachment: false,
+    structured_output: true,
+    temperature: true,
+    modalities_input: JSON.stringify(["text"]),
+    modalities_output: JSON.stringify(["text"]),
+    knowledge_cutoff: null,
+    release_date: null,
+    last_updated: null,
+    status: null,
+    family: null,
+    open_weights: false,
+    limit_context: limitContext,
+    limit_input: limitContext,
+    limit_output: 4096,
+    interleaved_field: null,
+  };
+}
+
+function target(modelStr: string) {
+  return {
+    kind: "model" as const,
+    stepId: modelStr,
+    executionKey: modelStr,
+    modelStr,
+    provider: modelStr.includes("/") ? modelStr.split("/")[0] : modelStr,
+    providerId: null,
+    connectionId: null,
+    weight: 1,
+    label: null,
+  };
+}
+
+// A generic Responses-API body whose estimate lands near `tokens` tokens (4 chars/token).
+// Uses `input:` (not `messages:`) to mirror the OpenCode/Codex Responses surface.
+function bigResponsesBody(tokens: number) {
+  return { input: [["user", "x".repeat(tokens * 4)]] };
+}
+
+const noopLog = { info() {}, warn() {}, error() {}, debug() {} };
+
+test("#10225 getKnownContextOverflow defers the hard overflow when compression is available", () => {
+  saveModelsDevCapabilities({ codex: { "gpt-5.6-terra": capabilityEntry(272_000) } });
+  const body = bigResponsesBody(275_000);
+
+  // Compression enabled + target can compress -> defer (null).
+  assert.equal(
+    getKnownContextOverflow([target("codex/gpt-5.6-terra")], body, {
+      deferContextOverflowWhenCompressible: true,
+    }),
+    null,
+    "compressible request must defer so chatCore compression can run (#10225)"
+  );
+
+  // Compression disabled -> the existing hard overflow is preserved (never lose #7177).
+  const hard = getKnownContextOverflow([target("codex/gpt-5.6-terra")], body);
+  assert.ok(hard);
+  assert.ok(hard.requiredContextTokens > hard.maxKnownContextTokens);
+
+  // Compression enabled but EVERY target is excluded from compression -> keep the hard gate.
+  const excluded = getKnownContextOverflow([target("codex/gpt-5.6-terra")], body, {
+    deferContextOverflowWhenCompressible: true,
+    compressionExclusions: ["gpt-5.6-terra"],
+  });
+  assert.ok(excluded, "fully-excluded targets must retain the hard preflight");
+});
+
+test("#10225 combo does not early-400 a compressible over-limit request when deferral is on", async () => {
+  saveModelsDevCapabilities({ codex: { "gpt-5.6-terra": capabilityEntry(272_000) } });
+  let dispatches = 0;
+
+  const response = await handleComboChat({
+    body: bigResponsesBody(275_000),
+    combo: {
+      name: "codex-compress-overflow",
+      strategy: "priority",
+      models: ["codex/gpt-5.6-terra"],
+    },
+    deferContextOverflowWhenCompressible: true,
+    clientManagedResponsesContext: false,
+    isModelAvailable: async () => true,
+    handleSingleModel: async () => {
+      dispatches += 1;
+      return new Response("ok", { status: 200 });
+    },
+    log: noopLog,
+  });
+
+  assert.notEqual(response.status, 400, "compression-enabled request must reach chatCore");
+  assert.equal(dispatches, 1, "must dispatch so chatCore compaction runs first");
+});
+
+test("#10225 combo keeps the fast 400 when compression is disabled", async () => {
+  saveModelsDevCapabilities({ codex: { "gpt-5.6-terra": capabilityEntry(272_000) } });
+  let dispatches = 0;
+
+  const response = await handleComboChat({
+    body: bigResponsesBody(275_000),
+    combo: {
+      name: "codex-compress-disabled",
+      strategy: "priority",
+      models: ["codex/gpt-5.6-terra"],
+    },
+    deferContextOverflowWhenCompressible: false,
+    clientManagedResponsesContext: false,
+    isModelAvailable: async () => true,
+    handleSingleModel: async () => {
+      dispatches += 1;
+      return new Response("ok", { status: 200 });
+    },
+    log: noopLog,
+  });
+
+  assert.equal(response.status, 400);
+  assert.equal(dispatches, 0, "#7177 anti-exhaustion guard must survive when compression is off");
+  const body = await response.json();
+  assert.equal(body.error.code, "context_length_exceeded");
+});
+
+// #10501-sweep #10503 — the deferral above is NOT target-aware by default: it only
+// checks operator-named compression exclusions, never whether chatCore will actually
+// attempt compression for the resolved target. handleChatCore.ts unconditionally sets
+// `compressionExcluded = nativeCodexPassthrough || ...` for a verified native Codex
+// Responses passthrough target (open-sse/handlers/chatCore.ts) — deferring the
+// preflight there means an oversized request sails past BOTH gates uncompressed. These
+// tests pin the fix: a native-codex-passthrough target must never count toward "can
+// compress", so the hard preflight stays active and no upstream dispatch happens.
+// NOTE on `clientManagedResponsesContext: false` below: these tests deliberately do
+// NOT set it, to isolate the fix from the PRE-EXISTING, unrelated early-return a few
+// lines above in knownContextOverflow.ts ("Native Codex Responses clients compact
+// their own item history") — that block ALSO returns null for an all-codex pool, but
+// only when `clientManagedResponsesContext === true` (a VERIFIED native client). The
+// bug this fix targets is broader: chatCore's `shouldUseNativeCodexPassthrough` short-
+// circuits to true for `provider === "codex"` regardless of verification (see
+// passthroughHelpers.ts), so an UNVERIFIED request that nonetheless targets a `codex`
+// combo member over `/v1/responses` in openai-responses format still hits chatCore's
+// compression bypass — exactly the gap `sourceFormat`/`endpointPath` (not the looser
+// `clientManagedResponsesContext` flag) now closes.
+test("#10503 getKnownContextOverflow REFUSES to defer when the only target is native Codex Responses passthrough", () => {
+  saveModelsDevCapabilities({ codex: { "gpt-5.6-terra": capabilityEntry(272_000) } });
+  const body = bigResponsesBody(275_000);
+
+  const overflow = getKnownContextOverflow([target("codex/gpt-5.6-terra")], body, {
+    deferContextOverflowWhenCompressible: true,
+    sourceFormat: "openai-responses",
+    endpointPath: "/v1/responses",
+  });
+
+  assert.ok(
+    overflow,
+    "a native-codex-passthrough target must never be treated as compressible — the " +
+      "hard preflight must stay active (chatCore disables compression for it entirely)"
+  );
+});
+
+test("#10503 getKnownContextOverflow still defers when a genuinely compressible sibling target is present", () => {
+  saveModelsDevCapabilities({
+    codex: { "gpt-5.6-terra": capabilityEntry(272_000) },
+    openai: { "gpt-5.6-terra": capabilityEntry(272_000) },
+  });
+  const body = bigResponsesBody(275_000);
+
+  // A heterogeneous pool where at least ONE target (openai) genuinely runs
+  // compression must still defer — deferral is a per-request decision, and other
+  // targets in the pool are unaffected by the codex-specific compression bypass.
+  const overflow = getKnownContextOverflow(
+    [target("codex/gpt-5.6-terra"), target("openai/gpt-5.6-terra")],
+    body,
+    {
+      deferContextOverflowWhenCompressible: true,
+      sourceFormat: "openai-responses",
+      endpointPath: "/v1/responses",
+    }
+  );
+
+  assert.equal(overflow, null, "a genuinely compressible sibling target must still defer");
+});
+
+test("#10503 handleComboChat: native-codex-passthrough pool fails FAST locally, zero upstream dispatches", async () => {
+  saveModelsDevCapabilities({ codex: { "gpt-5.6-terra": capabilityEntry(272_000) } });
+  let dispatches = 0;
+
+  const response = await handleComboChat({
+    body: bigResponsesBody(275_000),
+    combo: {
+      name: "codex-native-passthrough-overflow",
+      strategy: "priority",
+      models: ["codex/gpt-5.6-terra"],
+    },
+    deferContextOverflowWhenCompressible: true,
+    sourceFormat: "openai-responses",
+    endpointPath: "/v1/responses",
+    isModelAvailable: async () => true,
+    handleSingleModel: async () => {
+      dispatches += 1;
+      return new Response("ok", { status: 200 });
+    },
+    log: noopLog,
+  });
+
+  assert.equal(
+    response.status,
+    400,
+    "must fail fast locally instead of dispatching an oversized, uncompressible request"
+  );
+  assert.equal(dispatches, 0, "no wasted upstream call for a target that can never compress");
+  const responseBody = await response.json();
+  assert.equal(responseBody.error.code, "context_length_exceeded");
+});
+
+// #10503 item 2 — drive the REAL chatCore compression pipeline end-to-end (not just the
+// pure getKnownContextOverflow helper): a genuinely compressible multi-turn request must
+// have chatCore's proactive/last-resort compression actually run and dispatch the
+// COMPRESSED body upstream; a request that is STILL too large after compression must be
+// rejected locally with zero upstream dispatch (fail-fast, matching the codex-passthrough
+// case above in outcome, but via the "compression tried and wasn't enough" path instead
+// of "compression was never eligible").
+//
+// Uses an unregistered synthetic provider + CONTEXT_LENGTH_<PROVIDER> env override
+// (same technique as tests/unit/chatcore-combo-context-limit-8378.test.ts) so the
+// context limit is small and deterministic without depending on any real catalog entry.
+// Compression targets conversation HISTORY (older turns), not the current terminal
+// message — this is why the fixtures below build many small history turns plus one
+// short final turn (compressible case) vs one large, irreducible final turn
+// (still-too-large case).
+const CHATCORE_PROBE_PROVIDER = "combo10503probe";
+const CHATCORE_PROBE_MODEL = "combo10503probemodel";
+const CHATCORE_LIMIT_ENV = "CONTEXT_LENGTH_COMBO10503PROBE";
+
+function buildHistoryBody(turns: number, finalMessageChars: number) {
+  const messages: Array<{ role: string; content: string }> = [
+    { role: "system", content: "You are a helpful assistant." },
+  ];
+  for (let i = 0; i < turns; i++) {
+    messages.push({
+      role: "user",
+      content: `Message number ${i}: Alpha bravo charlie delta echo foxtrot golf hotel india juliet kilo lima mno pqr stu.`,
+    });
+    messages.push({
+      role: "assistant",
+      content: `Reply number ${i}: verbose filler answer with extra padding text for realism.`,
+    });
+  }
+  messages.push({ role: "user", content: "FINAL: " + "z".repeat(finalMessageChars) });
+  return { model: CHATCORE_PROBE_MODEL, messages, stream: false };
+}
+
+async function invokeChatCoreCapturingUpstream(body: Record<string, unknown>) {
+  const originalFetch = globalThis.fetch;
+  let dispatched = false;
+  let sentBodyJson: string | null = null;
+  globalThis.fetch = async (_url: RequestInfo | URL, init: RequestInit = {}) => {
+    dispatched = true;
+    sentBodyJson = init.body ? String(init.body) : null;
+    return new Response(
+      JSON.stringify({
+        id: "chatcmpl-10503",
+        object: "chat.completion",
+        model: CHATCORE_PROBE_MODEL,
+        choices: [
+          { index: 0, message: { role: "assistant", content: "ok" }, finish_reason: "stop" },
+        ],
+        usage: { prompt_tokens: 1, completion_tokens: 1, total_tokens: 2 },
+      }),
+      { status: 200, headers: { "content-type": "application/json" } }
+    );
+  };
+  try {
+    const result = await handleChatCore({
+      body,
+      modelInfo: {
+        provider: CHATCORE_PROBE_PROVIDER,
+        model: CHATCORE_PROBE_MODEL,
+        extendedContext: false,
+      },
+      credentials: { apiKey: "sk-test", providerSpecificData: {} },
+      log: { debug() {}, info() {}, warn() {}, error() {} },
+      clientRawRequest: {
+        endpoint: "/v1/chat/completions",
+        body,
+        headers: new Headers({ accept: "application/json" }),
+      },
+      userAgent: "unit-test",
+    } as never);
+    return { result, dispatched, sentBodyJson };
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+}
+
+test("#10503 real chatCore path: a compressible request dispatches the COMPRESSED (not raw) body upstream", async () => {
+  const originalEnv = process.env[CHATCORE_LIMIT_ENV];
+  process.env[CHATCORE_LIMIT_ENV] = "500";
+  await updateCompressionSettings({
+    enabled: true,
+    defaultMode: "standard",
+    autoTriggerTokens: 1,
+    autoTriggerMode: "standard",
+    engines: { rtk: { enabled: true }, caveman: { enabled: true } },
+  } as never);
+  try {
+    const body = buildHistoryBody(150, 50);
+    const rawLen = JSON.stringify(body.messages).length;
+
+    const { dispatched, sentBodyJson } = await invokeChatCoreCapturingUpstream(body);
+
+    assert.ok(dispatched, "compression must let a genuinely compressible request reach chatCore's dispatch");
+    assert.ok(sentBodyJson, "the dispatched request must carry a body");
+    assert.ok(
+      sentBodyJson!.length < rawLen * 0.5,
+      `expected the DISPATCHED body (${sentBodyJson!.length} chars) to be substantially ` +
+        `smaller than the raw request (${rawLen} chars) — proves compression actually ran ` +
+        `and its output (not the raw body) is what reached upstream`
+    );
+  } finally {
+    if (originalEnv === undefined) delete process.env[CHATCORE_LIMIT_ENV];
+    else process.env[CHATCORE_LIMIT_ENV] = originalEnv;
+  }
+});
+
+test("#10503 real chatCore path: STILL too large after compression → local rejection, ZERO upstream dispatch", async () => {
+  const originalEnv = process.env[CHATCORE_LIMIT_ENV];
+  process.env[CHATCORE_LIMIT_ENV] = "50";
+  await updateCompressionSettings({
+    enabled: true,
+    defaultMode: "standard",
+    autoTriggerTokens: 1,
+    autoTriggerMode: "standard",
+    engines: { rtk: { enabled: true }, caveman: { enabled: true } },
+  } as never);
+  try {
+    // The final turn alone (1000 chars, irreducible — compression trims HISTORY, not
+    // the current terminal message) already exceeds the 50-token limit, so no amount
+    // of history compaction can make this fit.
+    const body = buildHistoryBody(150, 1000);
+
+    const { result, dispatched } = await invokeChatCoreCapturingUpstream(body);
+
+    assert.equal(
+      dispatched,
+      false,
+      "fail-fast: a request that cannot fit even after compression must never reach fetch()"
+    );
+    assert.equal((result as { success: boolean }).success, false);
+    const failure = result as { success: false; error?: string; rawMessage?: string };
+    const message = failure.rawMessage ?? failure.error ?? "";
+    assert.match(message, /exceeds/i);
+  } finally {
+    if (originalEnv === undefined) delete process.env[CHATCORE_LIMIT_ENV];
+    else process.env[CHATCORE_LIMIT_ENV] = originalEnv;
+  }
+});