diff --git a/src/features/claude-code-agent-loader/claude-model-mapper.ts b/src/features/claude-code-agent-loader/claude-model-mapper.ts
index 7737de5f7..f0c0f9a86 100644
--- a/src/features/claude-code-agent-loader/claude-model-mapper.ts
+++ b/src/features/claude-code-agent-loader/claude-model-mapper.ts
@@ -11,6 +11,7 @@ const CLAUDE_CODE_ALIAS_MAP = new Map<string, string>([
 
 function mapClaudeModelString(model: string | undefined): string | undefined {
   if (!model) return undefined
+  if (typeof model !== "string") return undefined
 
   const trimmed = model.trim()
   if (trimmed.length === 0) return undefined
diff --git a/src/shared/fallback-chain-from-models.test.ts b/src/shared/fallback-chain-from-models.test.ts
index 16690d12c..94c6851ee 100644
--- a/src/shared/fallback-chain-from-models.test.ts
+++ b/src/shared/fallback-chain-from-models.test.ts
@@ -395,3 +395,65 @@ describe("findMostSpecificFallbackEntry", () => {
     })
   })
 })
+
+// Regression: type-guard against non-string model field (issue #4145).
+// Crash signature was `model.trim is not a function` aborting session.processor
+// for every provider when a caller forwarded a FallbackModelObject where a
+// plain string was expected.
+describe("parseFallbackModelEntry: non-string input (issue #4145)", () => {
+  test("returns undefined when caller forwards an object instead of a string", () => {
+    //#given
+    const wrong = { model: "anthropic/claude-opus-4-7", variant: "high" } as unknown as string
+
+    //#when
+    const parsed = parseFallbackModelEntry(wrong, "anthropic")
+
+    //#then: must not throw, must reject the malformed entry
+    expect(parsed).toBeUndefined()
+  })
+
+  test("returns undefined for null and undefined", () => {
+    //#given
+    const nullInput = null as unknown as string
+    const undefinedInput = undefined as unknown as string
+
+    //#when / #then
+    expect(parseFallbackModelEntry(nullInput, "anthropic")).toBeUndefined()
+    expect(parseFallbackModelEntry(undefinedInput, "anthropic")).toBeUndefined()
+  })
+
+  test("parseFallbackModelObjectEntry returns undefined when nested model field is non-string", () => {
+    //#given: FallbackModelObject whose .model was somehow forwarded as a nested
+    //object instead of a flat string (issue #4145 reproduction).
+    const malformedObject = {
+      model: ({ model: "claude-opus-4-7" } as unknown) as string,
+      variant: "high",
+    }
+
+    //#when
+    const parsed = parseFallbackModelObjectEntry(malformedObject, "anthropic")
+
+    //#then: must not throw, must reject the malformed entry
+    expect(parsed).toBeUndefined()
+  })
+
+  test("buildFallbackChainFromModels skips entries whose model is a number", () => {
+    //#given
+    const fallbackModels = [
+      "openai/gpt-5.5",
+      (42 as unknown) as string,
+    ]
+
+    //#when
+    const chain = buildFallbackChainFromModels(fallbackModels, "openai")
+
+    //#then: only the valid string survives, the number is dropped without crashing
+    expect(chain).toEqual([
+      {
+        providers: ["openai"],
+        model: "gpt-5.5",
+        variant: undefined,
+      },
+    ])
+  })
+})
diff --git a/src/shared/fallback-chain-from-models.ts b/src/shared/fallback-chain-from-models.ts
index 12e615d76..248250e22 100644
--- a/src/shared/fallback-chain-from-models.ts
+++ b/src/shared/fallback-chain-from-models.ts
@@ -4,6 +4,9 @@ import { normalizeFallbackModels } from "./model-resolver"
 import { KNOWN_VARIANTS } from "./known-variants"
 
 function parseVariantFromModel(rawModel: string): { modelID: string; variant?: string } {
+  if (typeof rawModel !== "string") {
+    return { modelID: "" }
+  }
   const trimmedModel = rawModel.trim()
   if (!trimmedModel) {
     return { modelID: "" }
@@ -33,6 +36,7 @@ export function parseFallbackModelEntry(
   contextProviderID: string | undefined,
   defaultProviderID = "opencode",
 ): FallbackEntry | undefined {
+  if (typeof model !== "string") return undefined
   const trimmed = model.trim()
   if (!trimmed) return undefined
 
diff --git a/src/shared/model-string-parser.ts b/src/shared/model-string-parser.ts
index 220bbd880..b10cd989d 100644
--- a/src/shared/model-string-parser.ts
+++ b/src/shared/model-string-parser.ts
@@ -11,6 +11,9 @@ const KNOWN_VARIANTS = new Set([
 ])
 
 export function parseVariantFromModelID(rawModelID: string): { modelID: string; variant?: string } {
+  if (typeof rawModelID !== "string") {
+    return { modelID: "" }
+  }
   const trimmedModelID = rawModelID.trim()
   if (!trimmedModelID) {
     return { modelID: "" }
@@ -38,6 +41,7 @@ export function parseVariantFromModelID(rawModelID: string): { modelID: string;
 export function parseModelString(
   model: string,
 ): { providerID: string; modelID: string; variant?: string } | undefined {
+  if (typeof model !== "string") return undefined
   const trimmedModel = model.trim()
   if (!trimmedModel) return undefined
 
diff --git a/src/tools/delegate-task/model-string-parser.ts b/src/tools/delegate-task/model-string-parser.ts
index 820bb3cc3..a932ccd50 100644
--- a/src/tools/delegate-task/model-string-parser.ts
+++ b/src/tools/delegate-task/model-string-parser.ts
@@ -11,6 +11,9 @@ const KNOWN_VARIANTS = new Set([
 ])
 
 export function parseVariantFromModelID(rawModelID: string): { modelID: string; variant?: string } {
+  if (typeof rawModelID !== "string") {
+    return { modelID: "" }
+  }
   const trimmedModelID = rawModelID.trim()
   if (!trimmedModelID) {
     return { modelID: "" }
@@ -38,6 +41,7 @@ export function parseVariantFromModelID(rawModelID: string): { modelID: string;
 export function parseModelString(
   model: string,
 ): { providerID: string; modelID: string; variant?: string } | undefined {
+  if (typeof model !== "string") return undefined
   const trimmedModel = model.trim()
   if (!trimmedModel) return undefined