diff --git a/CHANGELOG.md b/CHANGELOG.md
index a2dd2608ff..7a814d2629 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@
 
 - **fix(api):** relay worker now binds the SSRF guard to a stable `const` name so minified standalone (Docker) builds resolve it ([#6149](https://github.com/diegosouzapw/OmniRoute/issues/6149)) — the Vercel/Deno relay generators embedded the shared `resolveRelayTarget` guard as a bare `${fn.toString()}` declaration while the worker body called the hardcoded literal name; SWC minification mangled the source function's name, so the deployed worker defined `<mangled>` but still called `resolveRelayTarget` → `ReferenceError`. Both templates now emit `const resolveRelayTarget = ${fn.toString()};` (the const name is a template literal, immune to minification). Regression guard: `tests/unit/relay-minified-fn-6149.test.ts` (4). (thanks @SeaXen)
 - **fix(providers):** refresh the stale NVIDIA NIM model registry — drop EOL `z-ai/glm-5.1`, add `z-ai/glm-5.2` and `nvidia/nemotron-3-ultra-550b-a55b` ([#6108](https://github.com/diegosouzapw/OmniRoute/issues/6108)). Regression guard: `tests/unit/nvidia-nim-registry-6108.test.ts`. (thanks @andrea-kingautomation)
+- **fix(backend):** GPT-family (codex) models now report a distinct `max_input_tokens` (272000) below their 400K `context_length` via an optional `maxInputTokens` on `RegistryModel`, so coding agents auto-compact correctly instead of overflowing the real input cap ([#6191](https://github.com/diegosouzapw/OmniRoute/issues/6191)). Regression guard: `tests/unit/gpt-max-input-tokens-6191.test.ts`. (thanks @luweiCN)
 - **fix(backend):** call logs now record a **reasoning source/char-count** (migration 116, `reasoning_source`/`reasoning_chars`) for models that emit `reasoning_content`/`<think>` but report zero reasoning tokens in usage, so `tokens_reasoning` no longer silently under-represents reasoning — cost math is unchanged (the priced `tokens_reasoning` stays usage-derived) ([#6187](https://github.com/diegosouzapw/OmniRoute/issues/6187)). Regression guard: `tests/unit/reasoning-token-source-6187.test.ts`. (thanks @andrea-kingautomation)
 - **fix(auth):** a stale/changed `STORAGE_ENCRYPTION_KEY` now surfaces as a clear **424 `storage_encryption_stale`** ("re-enter the API key") instead of a misleading "Auth failed: 401" — the connection's ciphertext failed to decrypt and was coerced to an empty Bearer, hiding the real cause ([#6148](https://github.com/diegosouzapw/OmniRoute/issues/6148)). Regression guard: `tests/unit/decrypt-stale-key-hint-6148.test.ts`. (thanks @chirag127)
 - **fix(backend):** memory injection now keeps the injected system message **first** for providers that require it (via a `PROVIDERS_SYSTEM_MUST_BE_FIRST` capability), instead of the cache-safe mid-array splice that made strict providers reject the request with a 400 ([#6135](https://github.com/diegosouzapw/OmniRoute/issues/6135)). Regression guard: `tests/unit/memory-system-first-6135.test.ts`.
diff --git a/open-sse/config/providers/registry/codex/index.ts b/open-sse/config/providers/registry/codex/index.ts
index 2418a2bfb6..d739e09829 100644
--- a/open-sse/config/providers/registry/codex/index.ts
+++ b/open-sse/config/providers/registry/codex/index.ts
@@ -28,11 +28,17 @@ export const codexProvider: RegistryEntry = {
     // 1.05M). Public refs : openai/codex#19208, #19319, #19464 ;
     // opencode#24171. max_output_tokens is stripped server-side
     // (litellm#21193, codex#4138) so 128K is informational only.
+    // The usable INPUT budget is smaller than the 400K window (part is
+    // reserved for output), so max_input_tokens must be distinct from
+    // context_length or coding agents never auto-compact (#6191). OpenAI's
+    // own live catalog reports ~272K for gpt-5.5 in Codex.
     {
       id: "gpt-5.5",
       name: "GPT 5.5",
       ...GPT_5_5_CODEX_CAPABILITIES,
       contextLength: 400000,
+      // #6191: input cap per reporter; TODO confirm exact value
+      maxInputTokens: 272000,
       maxOutputTokens: 128000,
     },
     {
@@ -40,6 +46,8 @@ export const codexProvider: RegistryEntry = {
       name: "GPT 5.5 (xHigh)",
       ...GPT_5_5_CODEX_CAPABILITIES,
       contextLength: 400000,
+      // #6191: input cap per reporter; TODO confirm exact value
+      maxInputTokens: 272000,
       maxOutputTokens: 128000,
     },
     {
@@ -47,6 +55,8 @@ export const codexProvider: RegistryEntry = {
       name: "GPT 5.5 (High)",
       ...GPT_5_5_CODEX_CAPABILITIES,
       contextLength: 400000,
+      // #6191: input cap per reporter; TODO confirm exact value
+      maxInputTokens: 272000,
       maxOutputTokens: 128000,
     },
     {
@@ -54,6 +64,8 @@ export const codexProvider: RegistryEntry = {
       name: "GPT 5.5 (Medium)",
       ...GPT_5_5_CODEX_CAPABILITIES,
       contextLength: 400000,
+      // #6191: input cap per reporter; TODO confirm exact value
+      maxInputTokens: 272000,
       maxOutputTokens: 128000,
     },
     {
@@ -61,6 +73,8 @@ export const codexProvider: RegistryEntry = {
       name: "GPT 5.5 (Low)",
       ...GPT_5_5_CODEX_CAPABILITIES,
       contextLength: 400000,
+      // #6191: input cap per reporter; TODO confirm exact value
+      maxInputTokens: 272000,
       maxOutputTokens: 128000,
     },
     {
diff --git a/open-sse/config/providers/shared.ts b/open-sse/config/providers/shared.ts
index ecc4fd088d..1a90d258e6 100644
--- a/open-sse/config/providers/shared.ts
+++ b/open-sse/config/providers/shared.ts
@@ -56,6 +56,13 @@ export interface RegistryModel {
   unsupportedParams?: readonly string[];
   /** Maximum context window in tokens */
   contextLength?: number;
+  /**
+   * Explicit maximum input-token budget, when it is smaller than the full
+   * context window (e.g. OAuth backends that reserve part of the window for
+   * output). When set, catalog/capability builders prefer this over deriving
+   * max_input_tokens from contextLength (#6191).
+   */
+  maxInputTokens?: number;
   /**
    * Interleaved-reasoning signal, mirroring models.dev's `interleaved_field`.
    * Set to "reasoning_content" for models whose upstream runs DeepSeek thinking
diff --git a/src/app/api/v1/models/catalog.ts b/src/app/api/v1/models/catalog.ts
index 664cc0087a..73733107af 100644
--- a/src/app/api/v1/models/catalog.ts
+++ b/src/app/api/v1/models/catalog.ts
@@ -292,9 +292,13 @@ export async function getUnifiedModelsResponse(
         registryContext ??
         specContext ??
         (getTokenLimit(providerId, modelId) || undefined);
-      const maxInputTokens = isPositiveFiniteNumber(synced?.limit_input)
+      const registryInputLimit = isPositiveFiniteNumber(registryModel?.maxInputTokens)
+        ? registryModel.maxInputTokens
+        : undefined;
+      const syncedInputLimit = isPositiveFiniteNumber(synced?.limit_input)
         ? synced.limit_input
-        : contextLength;
+        : undefined;
+      const maxInputTokens = registryInputLimit ?? syncedInputLimit ?? contextLength;
       const maxOutputTokens = isPositiveFiniteNumber(synced?.limit_output)
         ? synced.limit_output
         : isPositiveFiniteNumber(spec?.maxOutputTokens)
diff --git a/src/lib/modelCapabilities.ts b/src/lib/modelCapabilities.ts
index 866d073643..cae1173042 100644
--- a/src/lib/modelCapabilities.ts
+++ b/src/lib/modelCapabilities.ts
@@ -406,7 +406,11 @@ export function getResolvedModelCapabilities(input: CapabilityInput): ResolvedMo
     structuredOutput: synced?.structured_output ?? null,
     temperature: synced?.temperature ?? null,
     contextWindow,
-    maxInputTokens: authoritativeContextWindow ?? synced?.limit_input ?? contextWindow,
+    maxInputTokens:
+      (typeof registryModel?.maxInputTokens === "number" ? registryModel.maxInputTokens : null) ??
+      authoritativeContextWindow ??
+      synced?.limit_input ??
+      contextWindow,
     maxOutputTokens:
       synced?.limit_output ??
       (typeof registryModel?.maxOutputTokens === "number" ? registryModel.maxOutputTokens : null) ??
diff --git a/tests/unit/gpt-max-input-tokens-6191.test.ts b/tests/unit/gpt-max-input-tokens-6191.test.ts
new file mode 100644
index 0000000000..f89c42edb1
--- /dev/null
+++ b/tests/unit/gpt-max-input-tokens-6191.test.ts
@@ -0,0 +1,67 @@
+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";
+
+// #6191: GPT-family codex models advertised their full 400K context window as
+// BOTH context_length and max_input_tokens, so coding agents never triggered
+// auto-compaction. The real usable input budget is smaller (~272K). These tests
+// pin the distinct-input-cap behavior and guard the contextLength fallback for
+// models that do NOT declare an explicit maxInputTokens.
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-gpt-input-cap-6191-"));
+process.env.DATA_DIR = TEST_DATA_DIR;
+
+const core = await import("../../src/lib/db/core.ts");
+const modelCapabilities = await import("../../src/lib/modelCapabilities.ts");
+
+function resetStorage() {
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+  fs.mkdirSync(TEST_DATA_DIR, { recursive: true });
+}
+
+test.beforeEach(() => {
+  resetStorage();
+});
+
+test.after(() => {
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+test("codex gpt-5.5 reports max_input_tokens smaller than its context window (#6191)", () => {
+  const caps = modelCapabilities.getResolvedModelCapabilities("codex/gpt-5.5");
+  assert.equal(caps.contextWindow, 400000);
+  assert.equal(caps.maxInputTokens, 272000);
+  assert.ok(
+    (caps.maxInputTokens ?? 0) < (caps.contextWindow ?? 0),
+    "max_input_tokens must be strictly smaller than context_length so agents compact"
+  );
+});
+
+test("all codex gpt-5.5 effort variants carry the distinct input cap (#6191)", () => {
+  for (const modelId of [
+    "codex/gpt-5.5-xhigh",
+    "codex/gpt-5.5-high",
+    "codex/gpt-5.5-medium",
+    "codex/gpt-5.5-low",
+  ]) {
+    const caps = modelCapabilities.getResolvedModelCapabilities(modelId);
+    assert.equal(caps.contextWindow, 400000, modelId);
+    assert.equal(caps.maxInputTokens, 272000, modelId);
+  }
+});
+
+test("regression: a model without maxInputTokens still falls back to its context window", () => {
+  // codex gpt-5.4 declares no maxInputTokens, so max_input_tokens must equal
+  // the context window (the historical fallback) — no under-reporting.
+  const caps = modelCapabilities.getResolvedModelCapabilities("codex/gpt-5.4");
+  assert.ok((caps.contextWindow ?? 0) > 0, "gpt-5.4 should have a context window");
+  assert.equal(
+    caps.maxInputTokens,
+    caps.contextWindow,
+    "without an explicit input cap, max_input_tokens falls back to context_length"
+  );
+});
diff --git a/tests/unit/model-capabilities-registry.test.ts b/tests/unit/model-capabilities-registry.test.ts
index b6582a1ca6..c4be2f0e49 100644
--- a/tests/unit/model-capabilities-registry.test.ts
+++ b/tests/unit/model-capabilities-registry.test.ts
@@ -108,7 +108,8 @@ test("canonical model capability resolver lets exact synced metadata override gl
 
   const codexGpt55 = modelCapabilities.getResolvedModelCapabilities("codex/gpt-5.5");
   assert.equal(codexGpt55.contextWindow, 400000);
-  assert.equal(codexGpt55.maxInputTokens, 400000);
+  // #6191: max_input_tokens is a distinct, smaller cap than the context window.
+  assert.equal(codexGpt55.maxInputTokens, 272000);
   assert.equal(codexGpt55.maxOutputTokens, 128000);
   assert.equal(codexGpt55.supportsThinking, true);
   assert.equal(codexGpt55.supportsVision, true);