diff --git a/packages/ai/src/protocols/shared.ts b/packages/ai/src/protocols/shared.ts
index 9ec502de65cf..41b1d5c1f558 100644
--- a/packages/ai/src/protocols/shared.ts
+++ b/packages/ai/src/protocols/shared.ts
@@ -6,11 +6,13 @@ import { Headers, HttpClientRequest, HttpClientResponse } from "effect/unstable/
 import {
   InvalidProviderOutputError,
   InvalidRequestError,
+  UnsupportedOperationError,
   AIError,
   HttpContext,
   type ContentPart,
   type LLMRequest,
   type MediaPart,
+  type ProviderID,
   type TextPart,
   type ToolResultPart,
 } from "../schema/index.js"
@@ -254,6 +256,29 @@ export const invalidRequest = (message: string, cause?: unknown) =>
     reason: new InvalidRequestError({ message, cause }),
   })
 
+/**
+ * Canonical constructor for operations the selected route does not implement.
+ * Prefer this over `invalidRequest` when the failure is a missing route
+ * capability rather than a malformed caller input, so consumers can branch on
+ * `reason._tag` plus `reason.operation` instead of matching message text.
+ */
+export const unsupportedOperation = (input: {
+  readonly operation: string
+  readonly message: string
+  readonly provider?: ProviderID
+  readonly route?: string
+  readonly cause?: unknown
+}) =>
+  new AIError({
+    reason: new UnsupportedOperationError({
+      operation: input.operation,
+      message: input.message,
+      provider: input.provider,
+      route: input.route,
+      cause: input.cause,
+    }),
+  })
+
 export const imageResponse = Effect.fn("ProviderShared.imageResponse")(function* (
   route: string,
   name: string,
diff --git a/packages/ai/src/protocols/xai-responses.ts b/packages/ai/src/protocols/xai-responses.ts
index acd55f347dc3..00a059e10198 100644
--- a/packages/ai/src/protocols/xai-responses.ts
+++ b/packages/ai/src/protocols/xai-responses.ts
@@ -46,9 +46,12 @@ const adapter = {
 const decodeBody = ProviderShared.validateWith(Schema.decodeUnknownEffect(XAIResponsesBody))
 const fromRequest = Effect.fn("XAIResponses.fromRequest")(function* (request: LLMRequest) {
   if (request.providerOptions?.contextManagement !== undefined)
-    return yield* ProviderShared.invalidRequest(
-      "xAI requires explicit compaction through LLMClient.compact; automatic context management is not supported",
-    )
+    return yield* ProviderShared.unsupportedOperation({
+      operation: "in-band-compaction",
+      provider: request.model.provider,
+      route: request.model.route.id,
+      message: "xAI requires explicit compaction through LLMClient.compact; automatic context management is not supported",
+    })
   return yield* decodeBody(yield* OpenResponses.fromRequestWithAdapter(request, adapter))
 })
 
diff --git a/packages/ai/src/route/client.ts b/packages/ai/src/route/client.ts
index 8ad47c491f2b..c8665f6722a8 100644
--- a/packages/ai/src/route/client.ts
+++ b/packages/ai/src/route/client.ts
@@ -585,9 +585,12 @@ export const layer: Layer.Layer<Service, never, RequestExecutor.Service> = Layer
         Effect.suspend(() => {
           const operation = request.model.route.compact
           if (!operation)
-            return ProviderShared.invalidRequest(
-              `${request.model.provider}/${request.model.route.id} does not support explicit compaction`,
-            )
+            return ProviderShared.unsupportedOperation({
+              operation: "compact",
+              provider: request.model.provider,
+              route: request.model.route.id,
+              message: `${request.model.provider}/${request.model.route.id} does not support explicit compaction`,
+            })
           return operation(prepareRequest(request), executor, options)
         }),
     })
diff --git a/packages/ai/src/schema/errors.ts b/packages/ai/src/schema/errors.ts
index 79a8f7886910..9566cb331018 100644
--- a/packages/ai/src/schema/errors.ts
+++ b/packages/ai/src/schema/errors.ts
@@ -35,6 +35,21 @@ export class InvalidRequestError extends Schema.TaggedError<InvalidRequestError>
   },
 ) {}
 
+/**
+ * A caller-requested operation the selected route does not implement, such as
+ * explicit compaction on a route without a compact endpoint. Detected locally
+ * before any network I/O, so unlike transport or provider-output failures it
+ * never carries HTTP context from a provider round-trip.
+ */
+export class UnsupportedOperationError extends Schema.TaggedError<UnsupportedOperationError>(
+  "AI.Error.UnsupportedOperation",
+)("UnsupportedOperation", {
+  ...ReasonFields,
+  operation: Schema.String,
+  provider: Schema.optional(ProviderID),
+  route: Schema.optional(RouteID),
+}) {}
+
 export class NoRouteError extends Schema.TaggedError<NoRouteError>("AI.Error.NoRoute")("NoRoute", {
   ...ReasonFields,
   route: RouteID,
@@ -107,6 +122,7 @@ export class UnknownProviderError extends Schema.TaggedError<UnknownProviderErro
 
 export const AIErrorReason = Schema.Union([
   InvalidRequestError,
+  UnsupportedOperationError,
   NoRouteError,
   AuthenticationError,
   RateLimitError,
diff --git a/packages/ai/test/compaction.test.ts b/packages/ai/test/compaction.test.ts
index edfcac94cf44..71104ff92301 100644
--- a/packages/ai/test/compaction.test.ts
+++ b/packages/ai/test/compaction.test.ts
@@ -1,8 +1,10 @@
 import { expect, test } from "bun:test"
-import { Schema } from "effect"
+import { Effect, Schema } from "effect"
 import { CompactionPart, CompactionResponse, LLMEvent, LLMResponse, Message, ProviderID } from "../src/schema/index.js"
 import { LLM, LLMClient, LLMRequest, LanguageModel } from "../src/index.js"
 import { OpenAI, Anthropic } from "../src/providers.js"
+import { testEffect } from "./lib/effect.js"
+import { fixedResponse } from "./lib/http.js"
 
 test("runtime capability checks follow model and route updates", () => {
   const supported = OpenAI.configure({ apiKey: "test" }).responses("fixture")
@@ -75,3 +77,25 @@ test("tagged content and event guards accept both checkpoint representations", (
     expect(Schema.decodeSync(codec)(Schema.encodeSync(codec)(message))).toEqual(message)
   }
 })
+
+testEffect(fixedResponse("")).effect(
+  "explicit compaction on a route without a compact endpoint fails with UnsupportedOperation",
+  () =>
+    Effect.gen(function* () {
+      const request = LLM.request({
+        model: Anthropic.configure({ apiKey: "test" }).model("fixture"),
+        prompt: "hello",
+      })
+      expect(LLMClient.canCompact(request)).toBe(false)
+      const error = yield* LLMClient.compact(
+        request as unknown as Parameters<typeof LLMClient.compact>[0],
+      ).pipe(Effect.flip)
+      expect(error.reason._tag).toBe("UnsupportedOperation")
+      expect(error.message).toContain("does not support explicit compaction")
+      if (error.reason._tag === "UnsupportedOperation") {
+        expect(error.reason.operation).toBe("compact")
+        expect(error.reason.provider).toBe("anthropic")
+        expect(error.reason.route).toBe("anthropic-messages")
+      }
+    }),
+)
diff --git a/packages/ai/test/provider/explicit-compaction.test.ts b/packages/ai/test/provider/explicit-compaction.test.ts
index a9d07f06331a..070f55bd6766 100644
--- a/packages/ai/test/provider/explicit-compaction.test.ts
+++ b/packages/ai/test/provider/explicit-compaction.test.ts
@@ -130,16 +130,22 @@ for (const model of [
           }),
         ],
       })
-      for (const candidate of [
-        LLMRequest.update(request, {
-          tools: [
-            { name: "unsupported", description: "Generation only", inputSchema: {}, native: { unsupported: {} } },
-          ],
-        }),
-        LLMRequest.update(request, { providerOptions: { contextManagement: "invalid-generation-option" } }),
-      ]) {
+      for (const [candidate, tag] of [
+        [
+          LLMRequest.update(request, {
+            tools: [
+              { name: "unsupported", description: "Generation only", inputSchema: {}, native: { unsupported: {} } },
+            ],
+          }),
+          "InvalidRequest",
+        ],
+        [
+          LLMRequest.update(request, { providerOptions: { contextManagement: "invalid-generation-option" } }),
+          model.provider === "xai" ? "UnsupportedOperation" : "InvalidRequest",
+        ],
+      ] as const) {
         const error = yield* LLMClient.generate(candidate).pipe(Effect.flip)
-        expect(error.reason._tag).toBe("InvalidRequest")
+        expect(error.reason._tag).toBe(tag)
         const response = yield* LLMClient.compact(candidate)
         expect(response.replacement[0]?.content[0]?.type).toBe("compaction")
       }
@@ -361,8 +367,9 @@ testEffect(fixedResponse("must not execute")).effect("xAI rejects automatic comp
       { providerOptions: { contextManagement: [{ type: "compaction" }] } },
     )
     const error = yield* LLMClient.generate(request).pipe(Effect.flip)
-    expect(error.reason._tag).toBe("InvalidRequest")
+    expect(error.reason._tag).toBe("UnsupportedOperation")
     expect(error.message).toContain("LLMClient.compact")
+    if (error.reason._tag === "UnsupportedOperation") expect(error.reason.operation).toBe("in-band-compaction")
   }),
 )
 
@@ -428,7 +435,9 @@ for (const model of [
       Effect.gen(function* () {
         // @ts-expect-error Untyped callers must still receive the runtime capability error.
         const error = yield* LLMClient.compact(LLM.request({ model, prompt: "hello" })).pipe(Effect.flip)
-        expect(error.reason._tag).toBe("InvalidRequest")
+        expect(error.reason._tag).toBe("UnsupportedOperation")
+        expect(error.message).toContain("does not support explicit compaction")
+        if (error.reason._tag === "UnsupportedOperation") expect(error.reason.operation).toBe("compact")
       }),
   )
 }
diff --git a/packages/ai/test/schema.test.ts b/packages/ai/test/schema.test.ts
index 28325eda5fbe..10a45d012372 100644
--- a/packages/ai/test/schema.test.ts
+++ b/packages/ai/test/schema.test.ts
@@ -24,6 +24,7 @@ import {
   ToolResultValue,
   TransportError,
   UnknownProviderError,
+  UnsupportedOperationError,
   Usage,
 } from "../src/schema/index.js"
 import { ProviderShared } from "../src/protocols/shared.js"
@@ -276,6 +277,12 @@ test("AI errors serialize diagnostics only on their typed reason", () => {
 test("AI error reasons are tagged Errors with required messages", () => {
   const reasons = [
     new InvalidRequestError({ message: "Invalid request" }),
+    new UnsupportedOperationError({
+      message: "Unsupported operation",
+      operation: "compact",
+      provider: model.provider,
+      route: "fake-route",
+    }),
     new NoRouteError({
       message: "No route",
       route: RouteID.make("missing"),
@@ -293,6 +300,7 @@ test("AI error reasons are tagged Errors with required messages", () => {
   ]
   expect(reasons.map((reason) => reason._tag)).toEqual([
     "InvalidRequest",
+    "UnsupportedOperation",
     "NoRoute",
     "Authentication",
     "RateLimit",
diff --git a/packages/core/src/aisdk.ts b/packages/core/src/aisdk.ts
index 215d6dcdea8d..f1e0c5c3cdc1 100644
--- a/packages/core/src/aisdk.ts
+++ b/packages/core/src/aisdk.ts
@@ -522,7 +522,11 @@ function userPart(part: ContentPart): UserContent {
 function assistantPart(part: ContentPart): AssistantContent {
   switch (part.type) {
     case "compaction":
-      throw ProviderShared.invalidRequest("AI SDK routes cannot replay native provider compaction state")
+      throw ProviderShared.unsupportedOperation({
+        operation: "compaction-replay",
+        provider: part.provider,
+        message: "AI SDK routes cannot replay native provider compaction state",
+      })
     case "text":
       return [{ type: "text", text: part.text, providerOptions: metadataProviderOptions(part.providerMetadata) }]
     case "media":
diff --git a/packages/core/src/session/runner/retry.ts b/packages/core/src/session/runner/retry.ts
index 88002291936e..b67cb23de0cb 100644
--- a/packages/core/src/session/runner/retry.ts
+++ b/packages/core/src/session/runner/retry.ts
@@ -47,6 +47,7 @@ export function isRetryable(error: AIError) {
     case "QuotaExceeded":
     case "ContentPolicy":
     case "InvalidRequest":
+    case "UnsupportedOperation":
     case "NoRoute":
       return false
     default: {
diff --git a/packages/core/src/session/to-session-error.ts b/packages/core/src/session/to-session-error.ts
index d731a959c9a2..90a1f4ed9004 100644
--- a/packages/core/src/session/to-session-error.ts
+++ b/packages/core/src/session/to-session-error.ts
@@ -25,6 +25,8 @@ export function toSessionError(cause: unknown): SessionError.Error {
         return providerError("provider.invalid-output", cause.reason)
       case "InvalidRequest":
         return providerError("provider.invalid-request", cause.reason)
+      case "UnsupportedOperation":
+        return providerError("provider.unsupported-operation", cause.reason)
       case "NoRoute":
         return providerError("provider.no-route", cause.reason)
       case "UnknownProvider":
diff --git a/packages/core/test/aisdk.test.ts b/packages/core/test/aisdk.test.ts
index c59e4e7e8a64..9d890a8c719d 100644
--- a/packages/core/test/aisdk.test.ts
+++ b/packages/core/test/aisdk.test.ts
@@ -90,8 +90,9 @@ it.effect("rejects native provider compaction rather than silently dropping repl
         ],
       }),
     ).pipe(Effect.flip)
-    expect(error.reason._tag).toBe("InvalidRequest")
+    expect(error.reason._tag).toBe("UnsupportedOperation")
     expect(error.message).toContain("cannot replay")
+    if (error.reason._tag === "UnsupportedOperation") expect(error.reason.operation).toBe("compaction-replay")
   }),
 )
 
diff --git a/packages/core/test/session-error.test.ts b/packages/core/test/session-error.test.ts
index 4836feb2d2a9..8febfa53f2eb 100644
--- a/packages/core/test/session-error.test.ts
+++ b/packages/core/test/session-error.test.ts
@@ -4,6 +4,7 @@ import {
   ContentPolicyError,
   InvalidProviderOutputError,
   InvalidRequestError,
+  UnsupportedOperationError,
   AIError,
   NoRouteError,
   ModelID,
@@ -43,6 +44,18 @@ describe("toSessionError", () => {
       "provider.invalid-output",
     )
     expect(toSessionError(llm(new InvalidRequestError({ message: "request" }))).type).toBe("provider.invalid-request")
+    expect(
+      toSessionError(
+        llm(
+          new UnsupportedOperationError({
+            message: "no compact endpoint",
+            operation: "compact",
+            provider: ProviderID.make("provider"),
+            route: "route",
+          }),
+        ),
+      ),
+    ).toEqual({ type: "provider.unsupported-operation", message: "no compact endpoint" })
     expect(
       toSessionError(
         llm(
@@ -140,6 +153,7 @@ describe("toSessionError", () => {
       llm(new ContentPolicyError({ message: "blocked" })),
       llm(new InvalidProviderOutputError({ message: "output" })),
       llm(new InvalidRequestError({ message: "request" })),
+      llm(new UnsupportedOperationError({ message: "unsupported", operation: "compact" })),
       llm(
         new NoRouteError({
           message: "failed",
@@ -151,7 +165,7 @@ describe("toSessionError", () => {
     ]
 
     expect(eligible.map(SessionRunnerRetry.isRetryable)).toEqual([true, true, true, true])
-    expect(ineligible.map(SessionRunnerRetry.isRetryable)).toEqual([false, false, false, false, false, false])
+    expect(ineligible.map(SessionRunnerRetry.isRetryable)).toEqual([false, false, false, false, false, false, false])
   })
 
   test("retries transport failures only when delivery is absent or not sent", () => {