diff --git a/src/cli/__snapshots__/model-fallback.test.ts.snap b/src/cli/__snapshots__/model-fallback.test.ts.snap
index 2468aa4a3..df91e07b8 100644
--- a/src/cli/__snapshots__/model-fallback.test.ts.snap
+++ b/src/cli/__snapshots__/model-fallback.test.ts.snap
@@ -750,10 +750,6 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models when
     "explore": {
       "model": "github-copilot/gpt-5-mini",
     },
-    "hephaestus": {
-      "model": "github-copilot/gpt-5.3-codex",
-      "variant": "medium",
-    },
     "librarian": {
       "model": "github-copilot/claude-sonnet-4.5",
     },
@@ -786,16 +782,12 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models when
       "model": "github-copilot/gemini-3-pro-preview",
       "variant": "high",
     },
-    "deep": {
-      "model": "github-copilot/gpt-5.3-codex",
-      "variant": "medium",
-    },
     "quick": {
       "model": "github-copilot/claude-haiku-4.5",
     },
     "ultrabrain": {
-      "model": "github-copilot/gpt-5.3-codex",
-      "variant": "xhigh",
+      "model": "github-copilot/gemini-3-pro-preview",
+      "variant": "high",
     },
     "unspecified-high": {
       "model": "github-copilot/claude-sonnet-4.5",
@@ -824,10 +816,6 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models with
     "explore": {
       "model": "github-copilot/gpt-5-mini",
     },
-    "hephaestus": {
-      "model": "github-copilot/gpt-5.3-codex",
-      "variant": "medium",
-    },
     "librarian": {
       "model": "github-copilot/claude-sonnet-4.5",
     },
@@ -860,16 +848,12 @@ exports[`generateModelConfig fallback providers uses GitHub Copilot models with
       "model": "github-copilot/gemini-3-pro-preview",
       "variant": "high",
     },
-    "deep": {
-      "model": "github-copilot/gpt-5.3-codex",
-      "variant": "medium",
-    },
     "quick": {
       "model": "github-copilot/claude-haiku-4.5",
     },
     "ultrabrain": {
-      "model": "github-copilot/gpt-5.3-codex",
-      "variant": "xhigh",
+      "model": "github-copilot/gemini-3-pro-preview",
+      "variant": "high",
     },
     "unspecified-high": {
       "model": "github-copilot/claude-opus-4.6",
@@ -1285,7 +1269,7 @@ exports[`generateModelConfig mixed provider scenarios uses all fallback provider
       "model": "opencode/claude-haiku-4-5",
     },
     "hephaestus": {
-      "model": "github-copilot/gpt-5.3-codex",
+      "model": "opencode/gpt-5.3-codex",
       "variant": "medium",
     },
     "librarian": {
@@ -1321,14 +1305,14 @@ exports[`generateModelConfig mixed provider scenarios uses all fallback provider
       "variant": "high",
     },
     "deep": {
-      "model": "github-copilot/gpt-5.3-codex",
+      "model": "opencode/gpt-5.3-codex",
       "variant": "medium",
     },
     "quick": {
       "model": "github-copilot/claude-haiku-4.5",
     },
     "ultrabrain": {
-      "model": "github-copilot/gpt-5.3-codex",
+      "model": "opencode/gpt-5.3-codex",
       "variant": "xhigh",
     },
     "unspecified-high": {
diff --git a/src/cli/model-fallback-requirements.ts b/src/cli/model-fallback-requirements.ts
index 94bf1f14d..f3f43e60b 100644
--- a/src/cli/model-fallback-requirements.ts
+++ b/src/cli/model-fallback-requirements.ts
@@ -17,9 +17,9 @@ export const CLI_AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
   },
   hephaestus: {
     fallbackChain: [
-      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
+      { providers: ["openai", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
     ],
-    requiresProvider: ["openai", "github-copilot", "opencode"],
+    requiresProvider: ["openai", "opencode"],
   },
   oracle: {
     fallbackChain: [
@@ -100,14 +100,14 @@ export const CLI_CATEGORY_MODEL_REQUIREMENTS: Record<string, ModelRequirement> =
   },
   ultrabrain: {
     fallbackChain: [
-      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.3-codex", variant: "xhigh" },
+      { providers: ["openai", "opencode"], model: "gpt-5.3-codex", variant: "xhigh" },
       { providers: ["google", "github-copilot", "opencode"], model: "gemini-3-pro", variant: "high" },
       { providers: ["anthropic", "github-copilot", "opencode"], model: "claude-opus-4-6", variant: "max" },
     ],
   },
   deep: {
     fallbackChain: [
-      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
+      { providers: ["openai", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
       { providers: ["anthropic", "github-copilot", "opencode"], model: "claude-opus-4-6", variant: "max" },
       { providers: ["google", "github-copilot", "opencode"], model: "gemini-3-pro", variant: "high" },
     ],
@@ -131,7 +131,7 @@ export const CLI_CATEGORY_MODEL_REQUIREMENTS: Record<string, ModelRequirement> =
   "unspecified-low": {
     fallbackChain: [
       { providers: ["anthropic", "github-copilot", "opencode"], model: "claude-sonnet-4-5" },
-      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
+      { providers: ["openai", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
       { providers: ["google", "github-copilot", "opencode"], model: "gemini-3-flash" },
     ],
   },
diff --git a/src/cli/model-fallback.test.ts b/src/cli/model-fallback.test.ts
index b87e6e6aa..2a40d58ea 100644
--- a/src/cli/model-fallback.test.ts
+++ b/src/cli/model-fallback.test.ts
@@ -421,16 +421,15 @@ describe("generateModelConfig", () => {
       expect(result.agents?.hephaestus?.variant).toBe("medium")
     })
 
-    test("Hephaestus is created when Copilot is available (github-copilot provider connected)", () => {
+    test("Hephaestus is NOT created when only Copilot is available (gpt-5.3-codex unavailable on github-copilot)", () => {
       // #given
       const config = createConfig({ hasCopilot: true })
 
       // #when
       const result = generateModelConfig(config)
 
-      // #then
-      expect(result.agents?.hephaestus?.model).toBe("github-copilot/gpt-5.3-codex")
-      expect(result.agents?.hephaestus?.variant).toBe("medium")
+      // #then - hephaestus is omitted because gpt-5.3-codex is not available on github-copilot
+      expect(result.agents?.hephaestus).toBeUndefined()
     })
 
     test("Hephaestus is created when OpenCode Zen is available (opencode provider connected)", () => {
diff --git a/src/shared/model-requirements.test.ts b/src/shared/model-requirements.test.ts
index 72d3266be..2991775eb 100644
--- a/src/shared/model-requirements.test.ts
+++ b/src/shared/model-requirements.test.ts
@@ -168,14 +168,14 @@ describe("AGENT_MODEL_REQUIREMENTS", () => {
     expect(primary.providers[0]).toBe("opencode")
   })
 
-  test("hephaestus requires openai/github-copilot/opencode provider", () => {
+  test("hephaestus requires openai/opencode provider (not github-copilot since gpt-5.3-codex unavailable there)", () => {
     // #given - hephaestus agent requirement
     const hephaestus = AGENT_MODEL_REQUIREMENTS["hephaestus"]
 
     // #when - accessing hephaestus requirement
-    // #then - requiresProvider is set to openai, github-copilot, opencode (not requiresModel)
+    // #then - requiresProvider is set to openai and opencode only (github-copilot removed)
     expect(hephaestus).toBeDefined()
-    expect(hephaestus.requiresProvider).toEqual(["openai", "github-copilot", "opencode"])
+    expect(hephaestus.requiresProvider).toEqual(["openai", "opencode"])
     expect(hephaestus.requiresModel).toBeUndefined()
   })
 
@@ -497,3 +497,35 @@ describe("requiresModel field in categories", () => {
     expect(artistry.requiresModel).toBe("gemini-3-pro")
   })
 })
+
+describe("gpt-5.3-codex provider restrictions", () => {
+  test("no gpt-5.3-codex entry in AGENT_MODEL_REQUIREMENTS includes github-copilot as provider", () => {
+    // given - all agent requirements
+    const allAgentEntries = Object.values(AGENT_MODEL_REQUIREMENTS).flatMap(
+      (req) => req.fallbackChain
+    )
+
+    // when - filtering entries with gpt-5.3-codex model
+    const codexEntries = allAgentEntries.filter((entry) => entry.model === "gpt-5.3-codex")
+
+    // then - none of them include github-copilot as a provider
+    for (const entry of codexEntries) {
+      expect(entry.providers).not.toContain("github-copilot")
+    }
+  })
+
+  test("no gpt-5.3-codex entry in CATEGORY_MODEL_REQUIREMENTS includes github-copilot as provider", () => {
+    // given - all category requirements
+    const allCategoryEntries = Object.values(CATEGORY_MODEL_REQUIREMENTS).flatMap(
+      (req) => req.fallbackChain
+    )
+
+    // when - filtering entries with gpt-5.3-codex model
+    const codexEntries = allCategoryEntries.filter((entry) => entry.model === "gpt-5.3-codex")
+
+    // then - none of them include github-copilot as a provider
+    for (const entry of codexEntries) {
+      expect(entry.providers).not.toContain("github-copilot")
+    }
+  })
+})
diff --git a/src/shared/model-requirements.ts b/src/shared/model-requirements.ts
index d382fb4ed..9a795ba76 100644
--- a/src/shared/model-requirements.ts
+++ b/src/shared/model-requirements.ts
@@ -24,9 +24,9 @@ export const AGENT_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
   },
   hephaestus: {
     fallbackChain: [
-      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
+      { providers: ["openai", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
     ],
-    requiresProvider: ["openai", "github-copilot", "opencode"],
+    requiresProvider: ["openai", "opencode"],
   },
   oracle: {
     fallbackChain: [
@@ -101,14 +101,14 @@ export const CATEGORY_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
   },
   ultrabrain: {
     fallbackChain: [
-      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.3-codex", variant: "xhigh" },
+      { providers: ["openai", "opencode"], model: "gpt-5.3-codex", variant: "xhigh" },
       { providers: ["google", "github-copilot", "opencode"], model: "gemini-3-pro", variant: "high" },
       { providers: ["anthropic", "github-copilot", "opencode"], model: "claude-opus-4-6", variant: "max" },
     ],
   },
   deep: {
     fallbackChain: [
-      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
+      { providers: ["openai", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
       { providers: ["anthropic", "github-copilot", "opencode"], model: "claude-opus-4-6", variant: "max" },
       { providers: ["google", "github-copilot", "opencode"], model: "gemini-3-pro", variant: "high" },
     ],
@@ -132,7 +132,7 @@ export const CATEGORY_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
   "unspecified-low": {
     fallbackChain: [
       { providers: ["anthropic", "github-copilot", "opencode"], model: "claude-sonnet-4-6" },
-      { providers: ["openai", "github-copilot", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
+      { providers: ["openai", "opencode"], model: "gpt-5.3-codex", variant: "medium" },
       { providers: ["google", "github-copilot", "opencode"], model: "gemini-3-flash" },
     ],
   },