diff --git a/headroom/proxy/handlers/anthropic.py b/headroom/proxy/handlers/anthropic.py
index 9a5b120a3..3b795edf4 100644
--- a/headroom/proxy/handlers/anthropic.py
+++ b/headroom/proxy/handlers/anthropic.py
@@ -3501,6 +3501,20 @@ class AnthropicHandlerMixin:
                         body_mutation_tracker.mark_mutated(
                             "ccr_streaming_retrieve_buffered_non_stream"
                         )
+                    # The body now asks for a non-streaming reply, so the
+                    # client's ``Accept: text/event-stream`` no longer describes
+                    # the response being requested. Forwarding it unchanged
+                    # sends upstream a self-contradicting request: "answer as
+                    # JSON" in the body, "I only accept SSE" in the headers.
+                    #
+                    # Anthropic tolerates that. Stricter Anthropic-compatible
+                    # gateways do not: GitHub Copilot's returns a generic
+                    # ``api_error``, which is why a session's first call
+                    # succeeded and the next one — the first to carry a
+                    # redeemable marker, and so the first to be buffered —
+                    # failed (#3078).
+                    _accept_key = next((k for k in headers if k.lower() == "accept"), "accept")
+                    headers[_accept_key] = "application/json"
                     logger.info(
                         f"[{request_id}] CCR: stream:true request has "
                         "headroom_retrieve available; using buffered stream:false "
@@ -3903,10 +3917,16 @@ class AnthropicHandlerMixin:
                                         body_mutated=True,
                                     )
                                 )
+                                # A continuation is a non-streaming call, so it
+                                # needs a matching Accept for the same reason the
+                                # buffered flip above does (#3078).
                                 ccr_outbound_headers = {
-                                    **continuation_headers,
-                                    "content-type": "application/json",
+                                    k: v
+                                    for k, v in continuation_headers.items()
+                                    if k.lower() not in ("accept", "content-type")
                                 }
+                                ccr_outbound_headers["content-type"] = "application/json"
+                                ccr_outbound_headers["accept"] = "application/json"
                                 log_outbound_request(
                                     forwarder="anthropic_ccr_continuation",
                                     method="POST",
diff --git a/headroom/proxy/handlers/openai.py b/headroom/proxy/handlers/openai.py
index 29a11bcf3..992c76df5 100644
--- a/headroom/proxy/handlers/openai.py
+++ b/headroom/proxy/handlers/openai.py
@@ -5616,6 +5616,12 @@ class OpenAIHandlerMixin:
             if body.get("stream") is not False:
                 body["stream"] = False
                 body_mutation_tracker.mark_mutated("ccr_streaming_retrieve_buffered_non_stream")
+            # Same contradiction as the Anthropic path: the body now asks for a
+            # non-streaming reply while the client's Accept still says SSE. This
+            # handler serves GitHub Copilot (see apply_copilot_api_auth below),
+            # whose gateway is one of the strict ones (#3078).
+            _accept_key = next((k for k in headers if k.lower() == "accept"), "accept")
+            headers[_accept_key] = "application/json"
             logger.info(
                 f"[{request_id}] CCR: stream:true /v1/responses request has "
                 "headroom_retrieve available; using buffered stream:false "
diff --git a/tests/test_buffered_ccr_accept_header.py b/tests/test_buffered_ccr_accept_header.py
new file mode 100644
index 000000000..3a663cb62
--- /dev/null
+++ b/tests/test_buffered_ccr_accept_header.py
@@ -0,0 +1,238 @@
+"""A buffered CCR turn must not ask for SSE it no longer wants (#3078).
+
+Server-side retrieval flips a ``stream: true`` turn to ``stream: false`` so the
+whole reply is in hand before answering. The body was rewritten; the client's
+``Accept: text/event-stream`` was not, so the request that went on the wire
+contradicted itself — "answer as JSON" in the body, "I only accept SSE" in the
+headers.
+
+Anthropic tolerates that, which is why it never showed up against the first-party
+API. GitHub Copilot's Anthropic-compatible gateway does not, and answers with a
+generic ``api_error``. That produced the reported shape exactly: the first call
+of an OpenCode session succeeds (no marker yet, so no buffering), and the next
+one — the first to carry a redeemable marker, and so the first to be flipped —
+fails.
+"""
+
+from __future__ import annotations
+
+import json
+
+import pytest
+
+fastapi = pytest.importorskip("fastapi")
+httpx = pytest.importorskip("httpx")
+
+from fastapi.testclient import TestClient  # noqa: E402
+
+from headroom.cache.backends import InMemoryBackend  # noqa: E402
+from headroom.cache.compression_store import (  # noqa: E402
+    get_compression_store,
+    reset_compression_store,
+)
+from headroom.ccr.tool_injection import create_ccr_tool_definition  # noqa: E402
+from headroom.proxy.server import ProxyConfig, create_app  # noqa: E402
+
+
+@pytest.fixture(autouse=True)
+def _store():
+    reset_compression_store()
+    get_compression_store(backend=InMemoryBackend())
+    try:
+        yield
+    finally:
+        reset_compression_store()
+
+
+def _drive(*, with_marker: bool, accept: str | None) -> dict[str, object]:
+    """Run one streaming turn; report the body `stream` and headers sent upstream."""
+    marker = get_compression_store().store(
+        original=json.dumps({"earlier": "tool output"}),
+        compressed="{}",
+        original_item_count=400,
+    )
+    config = ProxyConfig(
+        optimize=False,
+        cache_enabled=False,
+        rate_limit_enabled=False,
+        memory_enabled=False,
+        ccr_inject_tool=True,
+        ccr_handle_responses=True,
+        ccr_context_tracking=False,
+        image_optimize=False,
+    )
+    seen: dict[str, object] = {}
+    app = create_app(config)
+    with TestClient(app) as client:
+        proxy = client.app.state.proxy
+
+        async def _fake_retry(method, url, headers, body, stream=False, **kwargs):  # noqa: ANN001
+            sent = json.loads(body) if isinstance(body, (str, bytes)) else body
+            seen["stream"] = sent.get("stream")
+            seen["headers"] = dict(headers or {})
+            return httpx.Response(
+                200,
+                json={
+                    "id": "msg_1",
+                    "type": "message",
+                    "role": "assistant",
+                    "model": "claude-sonnet-4-6",
+                    "content": [{"type": "text", "text": "ok"}],
+                    "stop_reason": "end_turn",
+                    "usage": {
+                        "input_tokens": 10,
+                        "output_tokens": 5,
+                        "cache_read_input_tokens": 0,
+                        "cache_creation_input_tokens": 0,
+                    },
+                },
+            )
+
+        proxy._retry_request = _fake_retry  # type: ignore[assignment]
+
+        # A turn that is *not* flipped never reaches `_retry_request` — the plain
+        # streaming path has its own upstream call — so capture that one too.
+        async def _fake_stream(url, headers, body, *args, **kwargs):  # noqa: ANN001
+            from fastapi.responses import StreamingResponse
+
+            sent = json.loads(body) if isinstance(body, (str, bytes)) else body
+            seen["stream"] = sent.get("stream")
+            seen["headers"] = dict(headers or {})
+
+            async def _gen():
+                yield b'event: message_stop\ndata: {"type":"message_stop"}\n\n'
+
+            return StreamingResponse(_gen(), media_type="text/event-stream")
+
+        proxy._stream_response = _fake_stream  # type: ignore[assignment]
+        headers = {"x-api-key": "test-key", "anthropic-version": "2023-06-01"}
+        if accept is not None:
+            headers["accept"] = accept
+        content = f"go <<ccr:{marker}>>" if with_marker else "go"
+        client.post(
+            "/v1/messages",
+            json={
+                "model": "claude-sonnet-4-6",
+                "max_tokens": 64,
+                "stream": True,
+                "tools": [create_ccr_tool_definition("anthropic")],
+                "messages": [{"role": "user", "content": content}],
+            },
+            headers=headers,
+        )
+    return seen
+
+
+def _accepts(headers: dict) -> list[str]:
+    return [v for k, v in headers.items() if k.lower() == "accept"]
+
+
+def test_buffered_turn_asks_for_json() -> None:
+    seen = _drive(with_marker=True, accept="text/event-stream")
+
+    # Precondition: this turn really was flipped to buffered.
+    assert seen["stream"] is False
+    assert _accepts(seen["headers"]) == ["application/json"]  # type: ignore[arg-type]
+
+
+def test_buffered_turn_leaves_exactly_one_accept_header() -> None:
+    """Replaced, never appended — two Accept values is its own bug."""
+    seen = _drive(with_marker=True, accept="text/event-stream")
+
+    assert len(_accepts(seen["headers"])) == 1  # type: ignore[arg-type]
+
+
+def test_accept_header_is_replaced_regardless_of_casing() -> None:
+    """Header names are case-insensitive; the SSE value must not survive."""
+    seen = _drive(with_marker=True, accept="TEXT/EVENT-STREAM")
+
+    values = _accepts(seen["headers"])  # type: ignore[arg-type]
+    assert values == ["application/json"]
+    assert not any("event-stream" in v.lower() for v in values)
+
+
+def test_buffered_turn_without_a_client_accept_still_asks_for_json() -> None:
+    seen = _drive(with_marker=True, accept=None)
+
+    assert seen["stream"] is False
+    assert _accepts(seen["headers"]) == ["application/json"]  # type: ignore[arg-type]
+
+
+def test_a_streaming_turn_keeps_its_sse_accept() -> None:
+    """No marker means no flip, so nothing about the request should change."""
+    seen = _drive(with_marker=False, accept="text/event-stream")
+
+    assert seen["stream"] is not False
+    assert _accepts(seen["headers"]) == ["text/event-stream"]  # type: ignore[arg-type]
+
+
+# --------------------------------------------------------------------------- #
+# The same flip exists on the OpenAI Responses path, which serves Copilot
+# --------------------------------------------------------------------------- #
+def _drive_responses(*, accept: str) -> dict[str, object]:
+    """Run one streaming /v1/responses turn and report what went upstream."""
+    from headroom.ccr import CCR_TOOL_NAME
+
+    config = ProxyConfig(
+        optimize=False,
+        cache_enabled=False,
+        rate_limit_enabled=False,
+        memory_enabled=False,
+        ccr_inject_tool=True,
+        ccr_handle_responses=True,
+        ccr_context_tracking=False,
+        image_optimize=False,
+    )
+    seen: dict[str, object] = {}
+    app = create_app(config)
+    with TestClient(app) as client:
+        proxy = client.app.state.proxy
+
+        async def _fake_retry(method, url, headers, body, stream=False, **kwargs):  # noqa: ANN001
+            sent = json.loads(body) if isinstance(body, (str, bytes)) else body
+            seen["stream"] = sent.get("stream")
+            seen["headers"] = dict(headers or {})
+            return httpx.Response(
+                200,
+                json={
+                    "id": "resp_1",
+                    "object": "response",
+                    "model": "gpt-4o",
+                    "status": "completed",
+                    "output": [
+                        {
+                            "type": "message",
+                            "role": "assistant",
+                            "content": [{"type": "output_text", "text": "ok"}],
+                        }
+                    ],
+                    "usage": {"input_tokens": 5, "output_tokens": 2, "total_tokens": 7},
+                },
+            )
+
+        proxy._retry_request = _fake_retry  # type: ignore[assignment]
+        client.post(
+            "/v1/responses",
+            json={
+                "model": "gpt-4o",
+                "stream": True,
+                # Responses tool defs are flat, not nested under "function".
+                "tools": [{"type": "function", "name": CCR_TOOL_NAME}],
+                "input": "go",
+            },
+            headers={
+                "authorization": "Bearer test-key",
+                "accept": accept,
+                "content-type": "application/json",
+            },
+        )
+    return seen
+
+
+def test_responses_buffered_turn_asks_for_json() -> None:
+    """This handler serves GitHub Copilot, the gateway that rejects the mismatch."""
+    seen = _drive_responses(accept="text/event-stream")
+
+    assert seen.get("stream") is False, "precondition: the turn must be buffered"
+    assert _accepts(seen["headers"]) == ["application/json"]  # type: ignore[arg-type]
+    assert len(_accepts(seen["headers"])) == 1  # type: ignore[arg-type]